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`` war and peace , '' a tome , a slog , the sort of book you should n't read in bed because if you fall asleep , it could give you a concussion , right ? only partly . `` war and peace '' is a long book , sure , but it 's also a thrilling examination of history , populated with some of the deepest , most realistic characters you 'll find anywhere . and if its length intimidates you , just image how poor tolstoy felt . in 1863 , he set out to write a short novel about a political dissident returning from exile in siberia . five years later , he had produced a 1,200 page epic featuring love stories , battlefields , bankruptcies , firing squads , religious visions , the burning of moscow , and a semi-domesticated bear , but no exile and no political dissidents . here 's how it happened . tolstoy , a volcanic soul , was born to a famously eccentric aristocratic family in 1828 . by the time he was 30 , he had already dropped out of kazan university , gambled away the family fortune , joined the army , written memoirs , and rejected the literary establishment to travel europe . he then settled into yasnaya polyana , his ancestral mansion , to write about the return of the decembrists , a band of well-born revolutionaries pardoned in 1856 after 30 years in exile . but , tolstoy thought , how could he tell the story of the decembrists return from exile without telling the story of 1825 , when they revolted against the conservative tsar nicholas i ? and how could he do that without telling the story of 1812 , when napoleon 's disastrous invasion of russia helped trigger the authoritarianism the decembrists were rebelling against ? and how could he tell the story of 1812 without talking about 1805 , when the russians first learned of the threat napoleon posed after their defeat at the battle of austerlitz ? so tolstoy began writing , both about the big events of history and the small lives that inhabit those events . he focused on aristocrats , the class he knew best . the book only occasionally touches on the lives of the vast majority of the russian population , who were peasants , or even serfs , farmers bound to serve the owners of the land on which they lived . `` war and peace '' opens on the eve of war between france and russia . aristocrats at a cocktail party fret about the looming violence , but then change the topic to those things aristocrats always seem to care about : money , sex , and death . this first scene is indicative of the way the book bounces between the political and personal over an ever-widening canvas . there are no main characters in `` war and peace . '' instead , readers enter a vast interlocking web of relationships and questions . will the hapless and illegitimate son of a count marry a beautiful but conniving princess ? will his only friend survive the battlefields of austria ? and what about that nice young girl falling in love with both men at once ? real historical figures mix and mingle with all these fictional folk , napoleon appears several times , and even one of tolstoy 's ancestors plays a background part . but while the characters and their psychologies are gripping , tolstoy is not afraid to interrupt the narrative to pose insightful questions about history . why do wars start ? what are good battlefield tactics ? do nations rise and fall on the actions of so-called great men like napoleon , or are there larger cultural and economic forces at play ? these extended digressions are part of what make `` war and peace '' so panoramic in scope . but for some 19th century critics , this meant `` war and peace '' barely felt like a novel at all . it was a `` large , loose , baggy monster , '' in the words of henry james . tolstoy , in fact , agreed . to him , novels were a western european form . russian writers had to write differently because russian people lived differently . `` what is 'war and peace ' ? '' he asked . `` it is not a novel . still less an epic poem . still less a historical chronicle . 'war and peace ' is what the author wanted and was able to express in the form in which it was expressed . '' it is , in other words , the sum total of tolstoy 's imaginative powers , and nothing less . by the time `` war and peace '' ends , tolstoy has brought his characters to the year 1820 , 36 years before the events he originally hoped to write about . in trying to understand his own times , he had become immersed in the years piled up behind him . the result is a grand interrogation into history , culture , philosophy , psychology , and the human response to war .

russian writers had to write differently because russian people lived differently . `` what is 'war and peace ' ? '' he asked .

`` war and peace '' includes each of the following except :

good afternoon , ladies and gentlemen . let us welcome you to the final day of dramatic battle between great tragedians . it is a spring day here in ancient greece . nearly 17,000 patrons are filing into the theatre of dionysus to watch top playwrights , including favorites aeschylus and sophocles duke it out to see whose hero may be deemed most tragic , whose story most awful . well seacrestopolis , in last week 's battle of the choruses , all 50 members of each playwright 's chorus traveled back and forth across the stage , singing the strophe and antistrophe , telling misbegotten tales of woe . today 's first chorus is entering through the parados , taking their positions in the orchestra at the bottom of the stage . mario lopedokia , this is nothing we have n't seen before . all 50 members speaking from the depths of their souls . wait , what is this ? i 've not seen this before , seacrestopolis . there is one actor stepping out of choral formation , assuming an independent role in this play . can you make out who it is ? that looks like thespis . it seems he 's changing his mask , and taking on the role of another character . incredible . surely , thespis will go down in history as the very first actor . he has changed the face of theater forever . and that was just the warm-up act . on to the main attraction . aeschylus will have the stage first . let 's see what he does . we expect great things . last competition , sophocles beat him by a smidge , but aeschylus is still considered the father of tragedy . now , aeschylus frequently competes at this festival , the city dionysia . though his plays are violent , the bloodshed is never seen by the audience , which allows the dramatic tension to take center stage . let 's see what he does today to try to win his title back . here comes aeschylus 's chorus , but they seem to be missing quite a few people . what is going on here ? not only are they down a few people . there are two actors taking center stage . this is absolutely unheard of . he has build on thespis 's idea and added a second actor to the mix . aeschylus is relying on the two individuals to tell the story . the dialogue possible in tragedy now has taken precedence over the chorus . no wonder he drastically shrunk its size . this applause is well deserved . the crowd has hushed . sophocles 's actors and chorus are taking the stage for the play , `` oedipus rex . '' as usual , the chorus is set up in the orchestra . and what 's this ? sophocles has added a third actor . will this one-upmanship never end ? three actors , and they are changing their masks to take on several different roles as they weave the tale of oedipus , a nice fellow who kills his father and marries his mother . kills his father and marries his mother . that sounds pretty tragic to me . it is most tragic , mario lopedokia . call me crazy , but i 'm willing to bet that future generations will hold this play up as the perfect example of tragedy . excuse me , seacrestopolis . oedipus has left the stage after realizing jocasta was his wife and also his mother . where has he gone ? i ca n't even imagine . wait . the messenger has stepped on stage and is telling us of the great king 's actions . he says that oedipus , upon finding his mother , wife , whatever , jocasta , dead of her own hand in their incestuous bedroom , took the broaches from her dress and stabbed his eyes repeatedly . you ca n't blame the guy , can you ? bedded his mother , killed his father , is father and brother to his children . i might do the same . my friend , i do believe we 've seen it all . indeed , we have . there is nothing more tragic than oedipus . and sure enough , the judges who have been chosen by lot from all over greece are ready to announce the winner . oh , folks ! this is one for the history books . dark horse playwright , philocles , has taken first prize . what an upset . what a tragedy . what a night , folks . we have witnessed the laying of the foundation of modern theater and some great innovations : the shrinking of the chorus , the addition of three actors , and such catharsis . does n't a great tragedy just make you feel renewed and cleansed ? it sure does , but now we are out of time . i 'm seacrestopolis , and i 'm mario lopedokia . peace , love and catharsis .

the crowd has hushed . sophocles 's actors and chorus are taking the stage for the play , `` oedipus rex . '' as usual , the chorus is set up in the orchestra .

actors played multiple parts in each play .

dear nickelodeon , i 've gotten over how spongebob 's pants are not actually square . i can ignore most of the time that gary 's shell is not a logarithmic spiral . but what i can not forgive is that spongebob 's pineapple house is a mathematical impossibility . there 's three easy ways to find spirals on a pineapple . there 's the ones that wind up it going right , the ones that spiral up to the left , and the ones that go almost straight up -- keyword almost . if you count the number of spirals going left and the number of spirals going right , they 'll be adjacent fibonacci numbers -- 3 and 5 , or 5 and 8 , 8 and 13 , or 13 and 21 . you claim that spongebob squarepants lives in a pineapple under the sea , but does he really ? a true pineapple would have fibonacci spiral , so let 's take a look . because these images of his house do n't let us pick it up and turn it around to count the number of spirals going around it , it might be hard to figure out whether it 's mathematically a pineapple or not . but there 's a huge clue in the third spiral , the one going upwards . in this pineapple there 's 8 to the right , 13 to the left . you can add those numbers together to get how many spirals are in the set spiraling steeply upwards . in this case , 21 . the three sets of spirals in any pineapple are pretty much always adjacent fibonacci numbers . the rare mutant cases might show lucas numbers or something , but it will always be three adjacent numbers in a series . what you 'll never have is the same number of spirals both ways . pineapples , unlike people , do n't have bilateral symmetry . you 'll never have that third spiral be not a spiral , but just a straight line going up a pineapple . yet , when we look at spongebob 's supposed pineapple under the sea , it clearly has lines of pineapple things going straight up . it clearly has bilateral symmetry . it clearly is not actually a pineapple at all , because no pineapple could possibly grow that way . nickelodeon , you need to take a long , hard look in the mirror and think about the way you 're misrepresenting the universe to your viewers . this kind of mathematical oversight is simply irresponsible . sincerely , vi hart .

you 'll never have that third spiral be not a spiral , but just a straight line going up a pineapple . yet , when we look at spongebob 's supposed pineapple under the sea , it clearly has lines of pineapple things going straight up . it clearly has bilateral symmetry .

what is the huge clue that spongebob 's pineapple is mathematically impossible ?

anyone who has played a claw machine can relate to the experience of having the claw perfectly positioned only to see it weakly graze the prize before pulling back up . `` no man ! '' it may seem like the machine is n't even trying . and well ... `` it is not your imagination , those claw machines are rigged ! '' there are a couple of beloved stuffed animals that i have that are from a claw machine , a koala and a bear . that is vox.com writer , phil edwards . `` i looked at the instruction guides for a few of the biggest claw games out there . take for example , the manual for black tie toys advanced crane machine . if you look at page 8 , section subheading claw strength you will see a horrifying piece of information . `` managing profit is made easy . simply input the coin value , the average value of the merchandise , and the profit level . the machine will automatically calculate when to send full strength to the claw . '' alright , so if it cost 50 cents to play the game , and the prize inside cost 7 dollars . to make a profit of 50 % full power will be sent to the claw only about once every 21 games or so . that sucks . they also randomize that winning game within a range so that players ca n't predict when exactly it will happen . and you might notice a subheading that says `` dropping skill '' they can program the machine to make you think you almost won . they taunt you with it . you see the stuffed animal flying in the air . and then it drops it . and that just ruins everything . so , most of the time claw machines are more like slot machines , than like skeeball or wack-a-mole . `` who 's in charge here ! '' `` the claw ! '' the question of whether claw machines are a game of skill or chance goes back decades . the earlier versions back in the 1930s had very little element of skill and were marketed as highly profitable for their owners . this was the depression era and people were desperate for ways to get money moving . during a crackdown on organzied crime in the 1950s federal law classified claw machines as gambling devices and prohibited the transporation of them across state lines . after those laws were relaxed in the 1970s newer claw machines from europe and asia spread throughout the united states . they actually started calling them `` skill cranes '' because the joystick gave players more precise control . but owners had increasing control over profits as well . and they 've been met with a patchwork of state and local laws and regulations . if machine operators want to make that claw really really unfair against the players , there 's not a lot stopping them . most of the regulations focus on the prize size , not the strength of the claw . that 's a reason that you might see fewer of the `` win a free ipad '' claw machines or `` win a free iphone '' claw machines around . and more of just old fashioned stuffed animals . it 's great if players know what they are up against . especially since sites like youtube have enabled claw machine enthusiast to broadcast their victories . like this guy . `` i 'm matt magnone . join me as i venture out and win as much crap as i can from claw machines ! '' my best outcome of this is not that all the claw machines go away . since i first wrote this article , i 've spent a dollar on claw machines ... and i 've lost . all i want for people to know is that they are not the problem . the claw machine is the problem . `` ah , you piece of crap ! ''

`` who 's in charge here ! '' `` the claw ! '' the question of whether claw machines are a game of skill or chance goes back decades .

most contemporary regulations on claw machines focus on the value of the prizes not the strength or programming of the claw . based on what you learned from this video , why do you think that is ?

before he turned physics upside down , a young albert einstein supposedly showed off his genius by devising a complex riddle involving this list of clues . can you resist tackling a brain teaser written by one of the smartest people in history ? let 's give it a shot . the world 's rarest fish has been stolen from the city aquarium . the police have followed the scent to a street with five identical looking houses . but they ca n't search all the houses at once , and if they pick the wrong one , the thief will know they 're on his trail . it 's up to you , the city 's best detective , to solve the case . when you arrive on the scene , the police tell you what they know . one : each house 's owner is of a different nationality , drinks a different beverage , and smokes a different type of cigar . two : each house 's interior walls are painted a different color . three : each house contains a different animal , one of which is the fish . after a few hours of expert sleuthing , you gather some clues . it may look like a lot of information , but there 's a clear logical path to the solution . solving the puzzle will be a lot like sudoku , so you may find it helpful to organize your information in a grid , like this . pause the video on the following screen to examine your clues and solve the riddle . answer in : 3 2 1 to start , you fill in the information from clues eight and nine . immediately , you also realize that since the norwegian is at the end of the street , there 's only one house next to him , which must be the one with the blue walls in clue fourteen . clue five says the green-walled house 's owner drinks coffee . it ca n't be the center house since you already know its owner drinks milk , but it also ca n't be the second house , which you know has blue walls . and since clue four says the green-walled house must be directly to the left of the white-walled one , it ca n't be the first or fifth house either . the only place left for the green-walled house with the coffee drinker is the fourth spot , meaning the white-walled house is the fifth . clue one gives you a nationality and a color . since the only column missing both these values is the center one , this must be the brit 's red-walled home . now that the only unassigned wall color is yellow , this must be applied to the first house , where clue seven says the dunhill smoker lives . and clue eleven tells you that the owner of the horse is next door , which can only be the second house . the next step is to figure out what the norwegian in the first house drinks . it ca n't be tea , clue three tells you that 's the dane . as per clue twelve , it ca n't be root beer since that person smokes bluemaster , and since you already assigned milk and coffee , it must be water . from clue fifteen , you know that the norwegian 's neighbor , who can only be in the second house , smokes blends . now that the only spot in the grid without a cigar and a drink is in the fifth column , that must be the home of the person in clue twelve . and since this leaves only the second house without a drink , the tea-drinking dane must live there . the fourth house is now the only one missing a nationality and a cigar brand , so the prince-smoking german from clue thirteen must live there . through elimination , you can conclude that the brit smokes pall mall and the swede lives in the fifth house , while clue six and clue two tell you that these two have a bird and a dog , respectively . clue ten tells you that the cat owner lives next to the blend-smoking dane , putting him in the first house . now with only one spot left on the grid , you know that the german in the green-walled house must be the culprit . you and the police burst into the house , catching the thief fish-handed . while that explanation was straightforward , solving puzzles like this often involves false starts and dead ends . part of the trick is to use the process of elimination and lots of trial and error to hone in on the right pieces , and the more logic puzzles you solve , the better your intuition will be for when and where there 's enough information to make your deductions . and did young einstein really write this puzzle ? probably not . there 's no evidence he did , and some of the brands mentioned are too recent . but the logic here is not so different from what you 'd use to solve equations with multiple variables , even those describing the nature of the universe .

immediately , you also realize that since the norwegian is at the end of the street , there 's only one house next to him , which must be the one with the blue walls in clue fourteen . clue five says the green-walled house 's owner drinks coffee . it ca n't be the center house since you already know its owner drinks milk , but it also ca n't be the second house , which you know has blue walls .

at what point was it possible to confidently claim that the german lived in the green house ?

[ music playing ] my goal is to not actually do any measurements other than using pie . so the entire thing is going to pie based . we get the circumference in the exact number of pies -- give or take -- we get the diameter in number of pies . divide one by the other , we get pi . [ music playing ] pi was historically rarely calculated this way , because it 's notoriously inaccurate to try and get -- i know , we 're idiots -- it 's notoriously inaccurate to try and calculate pi by measuring a circle . [ music playing ] to get any kind of accuracy on our final answer we have to be as precise as we can be . [ music playing ] this is a mild problem , because i want to go from the very edge of the circle . but as you can see , i 've positioned all the pies exactly on the line as if they 're little mini tangents . so i 'm going to have to move these two out and then i can start doing the diameter exactly on the circumference there , like that . [ music playing ] so that 's 84 and 1/3 . 264 and 2/3 pies around , we have the diameter , is 84 and 1/3 pie . to get pi , we just divide the circumference by the diameter . ok , and if we actually work out what that is , it equals -- that 's pretty good . pi from pies , we got 3.13834 , which is approximately 3.14 . so using pies , we 've got pi to be 3.14 . i am a very happy man right now . with the diameter , and if i know that ratio , i can just measure the diameter . that 's the easy bit to measure . the center point will be my pen , and the outside will be the chalk . so the pies , in theory , are a fairly consistent size . so they 're all -- yeah they 're about the same . ok. [ music playing ]

so the entire thing is going to pie based . we get the circumference in the exact number of pies -- give or take -- we get the diameter in number of pies . divide one by the other , we get pi .

knowing the small pies have a diameter of 10.5cm - what is the area of the circle created in this video ?

each time you take a step , 200 muscles work in unison to lift your foot , propel it forward , and set it down . it 's just one of the many thousands of tasks performed by the muscular system . this network of over 650 muscles covers the body and is the reason we can blink , smile , run , jump , and stand upright . it 's even responsible for the heart 's dependable thump . first , what exactly is the muscular system ? it 's made up of three main muscle types : skeletal muscle , which attaches via tendons to our bones , cardiac muscle , which is only found in the heart , and smooth muscle , which lines the blood vessels and certain organs , like the intestine and uterus . all three types are made up of muscle cells , also known as fibers , bundled tightly together . these bundles receive signals from the nervous system that contract the fibers , which in turn generates force and motion . this produces almost all the movements we make . some of the only parts of the body whose motions are n't governed by the muscular system are sperm cells , the hair-like cilia in our airways , and certain white blood cells . muscle contraction can be split into three main types . the first two , shortening muscle fibers and lengthening them , generate opposing forces . so the biceps will shorten while the triceps will lengthen or relax , pulling up the arm and making it bend at the elbow . this allows us to , say , pick up a book , or if the muscle relationship is reversed , put it down . this complementary partnership exists throughout the muscular system . the third type of contraction creates a stabilizing force . in these cases , the muscle fibers do n't change in length , but instead keep the muscles rigid . this allows us to grip a mug of coffee or lean against a wall . it also maintains our posture by holding us upright . skeletal muscles form the bulk of the muscular system , make up about 30-40 % of the body 's weight , and generate most of its motion . some muscles are familiar to us , like the pectorals and the biceps . others may be less so , like the buccinator , a muscle that attaches your cheek to your teeth , or the body 's tiniest skeletal muscle , a one-millimeter-long tissue fragment called the stapedius that 's nestled deep inside the ear . wherever they occur , skeletal muscles are connected to the somatic nervous system , which gives us almost complete control over their movements . this muscle group also contains two types of muscle fibers to refine our motions even further , slow-twitch and fast-twitch . fast-twitch fibers react instantly when triggered but quickly use up their energy and tire out . slow-twitch fibers , on the other hand , are endurance cells . they react and use energy slowly so they can work for longer periods . a sprinter will accumulate more fast-twitch muscles in her legs through continuous practice , enabling her to quickly , if briefly , pick up the pace , whereas back muscles contain more slow-twitch muscles to maintain your posture all day . unlike the skeletal muscles , the body 's cardiac and smooth muscles are managed by the autonomic nervous system beyond our direct control . that makes your heart thump roughly 3 billion times over the course of your life , which supplies the body with blood and oxygen . autonomic control also contracts and relaxes smooth muscle in a rhythmic cycle . that pumps blood through the smooth internal walls of blood vessels , enables the intestine to constrict and push food through the digestive system , and allows the uterus to contract when a person is giving birth . as muscles work , they also use energy and produce an important byproduct , heat . in fact , muscle provides about 85 % of your warmth , which the heart and blood vessels then spread evenly across the body via the blood . without that , we could n't maintain the temperature necessary for our survival . the muscular system may be largely invisible to us , but it leaves its mark on almost everything we do , whether it 's the blink of an eye or a race to the finish line .

it 's even responsible for the heart 's dependable thump . first , what exactly is the muscular system ? it 's made up of three main muscle types : skeletal muscle , which attaches via tendons to our bones , cardiac muscle , which is only found in the heart , and smooth muscle , which lines the blood vessels and certain organs , like the intestine and uterus .

without the muscular system , we wouldn ’ t be able to make most of the body heat that we require .

this might seem hard to believe , but right now , 300 million women across the planet are experiencing the same thing : a period . the monthly menstrual cycle that leads to the period is a reality most women on earth will go through in their lives . but why is this cycle so universal ? and what makes it a cycle in the first place ? periods last anywhere between two and seven days , arising once within in a 28-day rotation . that whole system occurs on repeat , happening approximately 450 times during a woman 's life . behind the scenes are a series of hormonal controls that fine tune the body 's internal workings to make menstruation start or stop during those 28 days . this inner machinery includes two ovaries stocked with thousands of tiny sacks called follicles that each contain one oocyte , an unfertilized egg cell . at puberty , ovaries hold over 400 thousand egg cells , but release only one each month , which results in pregnancy or a period . here 's how this cycle unfolds . each month beginning around puberty , the hormone-producing pituitary gland in the brain starts releasing two substances into the blood : follicle stimulating hormone and luteinizing hormone . when they reach the ovaries , they encourage the internal egg cells to grow and mature . the follicles respond by pumping out estrogen . the egg cells grow and estrogen levels peak , inhibiting the production of fsh , and telling the pituitary to pump out more lh . that causes only the most mature egg cell from one of the ovaries to burst out of the follicle and through the ovary wall . this is called ovulation , and it usually happens ten to sixteen days before the start of a period . the tiny oocyte moves along the fallopian tube . a pregnancy can only occur if the egg is fertilized by a sperm cell within the next 24 hours . otherwise , the egg 's escapade ends , and the window for pregnancy closes for that month . meanwhile , the now empty follicle begins to release progesterone , another hormone that tells the womb 's lining to plump up with blood and nutrients in preparation for a fertilized egg that may embed there and grow . if it does n't embed , a few days later , the body 's progesterone and estrogen levels plummet , meaning the womb stops padding out and starts to degenerate , eventually falling away . blood and tissue leave the body , forming the period . the womb can take up to a week to clear out its unused contents , after which , the cycle begins anew . soon afterwards , the ovaries begin to secrete estrogen again , and the womb lining thickens , getting ready to accommodate a fertilized egg or be shed . hormones continually control these activities by circulating in ideal amounts delivered at just the right time . the cycle keeps on turning , transforming each day and each week into a milestone along its course towards pregnancy or a period . although this cycle appears to move by clockwork , there 's room for variation . women and their bodies are unique , after all . menstrual cycles occur at diffferent times in the month , ovulation comes at various points in the cycle , and some periods last longer than others . menstruation even begins and ends at different times in life for different women , too . in other words , variations between periods are normal . appreciating these differences and learning about this monthly process can empower women , giving them the tools to understand and take charge of their own bodies . that way , they 're able to factor this small cycle into a much larger cycle of life .

if it does n't embed , a few days later , the body 's progesterone and estrogen levels plummet , meaning the womb stops padding out and starts to degenerate , eventually falling away . blood and tissue leave the body , forming the period . the womb can take up to a week to clear out its unused contents , after which , the cycle begins anew .

every woman 's period is the same .

and you can see that the phosphorus has oxidized in the air and it ’ s taken some of the paper with it to generate a nice ‘ p ’ for phosphorus . so this is a sample of my favourite element , this is phosphorus . phosphorus is a very , very reactive element , especially this type of phosphorus which is white phosphorus . phosphorus itself , elemental phosphorus , exists as a number of different allotropes , so a bit like carbon with diamond and graphite , and here we have one of these allotropes ; it ’ s white phosphorus . it exists as discrete molecules of four phosphorus atoms connected like a pyramid , and those four phosphorus atoms are really strained , really reactive and they want to really react with oxygen as quick as they can . this is the sort that is used in phosphorus bombs . it burns very easily and gives a very intense white light out of it . so instantaneously you can see that fantastic oxidation reaction as the phosphorus reacts with the molecular oxygen in the gas inside the flask . you can put it out quite easily by putting water on it , provided that it hasn ’ t got too hot and the water will just evaporate . very reactive form the other allotropes , black and red phosphorus , are not as reactive and are much , much easier for us to handle . then there ’ s red phosphorus which is very much less reactive which is what is used in matches . it is used to make the heads of so-called ‘ non-safety ’ matches , which are the ones you can strike on any sort of material , on bricks and so on , and they tend to be this rather red colour here . so the phosphorus , you ’ ll notice , that it ’ s stored under water , and it ’ s stored under water to keep the oxygen and the atmosphere away because it reacts very vigorously and can burn , hence the signs , it ’ s very , very flammable . it ’ s so flammable in fact that it burns just with the heat of the skin . so you can see a number of scars across my hands , these are all from playing with phosphorus and doing reactions and experiments with phosphorus . phosphorus is a very important point inside your body . some of you may have read aldous huxley ’ s book called ‘ brave new world ’ , which when people died they were sent to the phosphorus reclamation plant . and each average size person contains somewhat over a pound of phosphorus inside their bodies , and nowadays people can use magnetic resonance imaging to look and see how the phosphorus is used inside the body . so i ’ ve taken a small sample of my white phosphorus and i ’ ve dissolved it in an organic solvent which is carbon disulphide , because the p4 is molecular and it dissolves really quite nicely in this molecular solvent . so now what we are going to do is we ’ re going to go outside and we ’ re going to put this on a piece of filter paper and then let the carbon disulphide evaporate so we can see the reaction of the phosphorus with the oxygen and the paper . and the average person produces inside their body about 70 kilos a day of a molecule called atp , adenosine triphosphate , which is the energy producing molecule inside the body . now you don ’ t keep 70 kilos inside you , or you would explode , but you ’ re using it up all the time but in the course of the day you synthesise nearly 70 kilos of this molecule . so as the carbon disulphide evaporates from the paper , well there ’ s a couple of drips . and you can see that the phosphorus has oxidized in the air and it ’ s taken some of the paper with it to generate a nice ‘ p ’ for phosphorus .

so this is a sample of my favourite element , this is phosphorus . phosphorus is a very , very reactive element , especially this type of phosphorus which is white phosphorus . phosphorus itself , elemental phosphorus , exists as a number of different allotropes , so a bit like carbon with diamond and graphite , and here we have one of these allotropes ; it ’ s white phosphorus .

in the exothermic oxidation of white phosphorus , a white product is formed . what is it ? and what is the acid that is generated when you breathe it in and it reacts with the water in your nose and mouth ?

in 1796 , the scientist edward jenner injected material from a cowpox virus into an eight-year-old boy with a hunch that this would provide the protection needed to save people from deadly outbreaks of the related smallpox virus . it was a success . the eight-year-old was inoculated against the disease and this became the first ever vaccine . but why did it work ? to understand how vaccines function , we need to know how the immune system defends us against contagious diseases in the first place . when foreign microbes invade us , the immune system triggers a series of responses in an attempt to identify and remove them from our bodies . the signs that this immune response is working are the coughing , sneezing , inflammation and fever we experience , which work to trap , deter and rid the body of threatening things , like bacteria . these innate immune responses also trigger our second line of defense , called adaptive immunity . special cells called b cells and t cells are recruited to fight microbes , and also record information about them , creating a memory of what the invaders look like , and how best to fight them . this know-how becomes handy if the same pathogen invades the body again . but despite this smart response , there 's still a risk involved . the body takes time to learn how to respond to pathogens and to build up these defenses . and even then , if a body is too weak or young to fight back when it 's invaded , it might face very serious risk if the pathogen is particularly severe . but what if we could prepare the body 's immune response , readying it before someone even got ill ? this is where vaccines come in . using the same principles that the body uses to defend itself , scientists use vaccines to trigger the body 's adaptive immune system , without exposing humans to the full strength disease . this has resulted in many vaccines , which each work uniquely , separated into many different types . first , we have live attenuated vaccines . these are made of the pathogen itself but a much weaker and tamer version . next , we have inactive vaccines , in which the pathogens have been killed . the weakening and inactivation in both types of vaccine ensures that pathogens do n't develop into the full blown disease . but just like a disease , they trigger an immune response , teaching the body to recognize an attack by making a profile of pathogens in preparation . the downside is that live attenuated vaccines can be difficult to make , and because they 're live and quite powerful , people with weaker immune systems ca n't have them , while inactive vaccines do n't create long-lasting immunity . another type , the subunit vaccine , is only made from one part of the pathogen , called an antigen , the ingredient that actually triggers the immune response . by even further isolating specific components of antigens , like proteins or polysaccharides , these vaccines can prompt specific responses . scientists are now building a whole new range of vaccines called dna vaccines . for this variety , they isolate the very genes that make the specific antigens the body needs to trigger its immune response to specific pathogens . when injected into the human body , those genes instruct cells in the body to make the antigens . this causes a stronger immune response , and prepares the body for any future threats , and because the vaccine only includes specific genetic material , it does n't contain any other ingredients from the rest of the pathogen that could develop into the disease and harm the patient . if these vaccines become a success , we might be able to build more effective treatments for invasive pathogens in years to come . just like edward jenner 's amazing discovery spurred on modern medicine all those decades ago , continuing the development of vaccines might even allow us to treat diseases like hiv , malaria , or ebola , one day .

by even further isolating specific components of antigens , like proteins or polysaccharides , these vaccines can prompt specific responses . scientists are now building a whole new range of vaccines called dna vaccines . for this variety , they isolate the very genes that make the specific antigens the body needs to trigger its immune response to specific pathogens .

dna vaccines and protein subunit vaccines can be designed and synthesized . what is the benefit of this ?

symmetry is everywhere in nature , and we usually associate it with beauty : a perfectly shaped leaf , or a butterfly with intricate patterns mirrored on each wing . but it turns out that asymmetry is pretty important , too , and more common than you might think , from crabs with one giant pincer claw to snail species whose shells ' always coil in the same direction . some species of beans only climb up their trellises clockwise , others , only counterclockwise , and even though the human body looks pretty symmetrical on the outside , it 's a different story on the inside . most of your vital organs are arranged asymmetrically . the heart , stomach , spleen , and pancreas lie towards the left . the gallbladder and most of your liver are on the right . even your lungs are different . the left one has two lobes , and the right one has three . the two sides of your brain look similar , but function differently . making sure this asymmetry is distributed the right way is critical . if all your internal organs are flipped , a condition called situs inversus , it 's often harmless . but incomplete reversals can be fatal , especially if the heart is involved . but where does this asymmetry come from , since a brand-new embryo looks identical on the right and left . one theory focuses on a small pit on the embryo called a node . the node is lined with tiny hairs called cilia , while tilt away from the head and whirl around rapidly , all in the same direction . this synchronized rotation pushes fluid from the right side of the embryo to the left . on the node 's left-hand rim , other cilia sense this fluid flow and activate specific genes on the embryo 's left side . these genes direct the cells to make certain proteins , and in just a few hours , the right and left sides of the embryo are chemically different . even though they still look the same , these chemical differences are eventually translated into asymmetric organs . asymmetry shows up in the heart first . it begins as a straight tube along the center of the embryo , but when the embryo is around three weeks old , the tube starts to bend into a c-shape and rotate towards the right side of the body . it grows different structures on each side , eventually turning into the familiar asymmetric heart . meanwhile , the other major organs emerge from a central tube and grow towards their ultimate positions . but some organisms , like pigs , do n't have those embryonic cilia and still have asymmetric internal organs . could all cells be intrinsically asymmetric ? probably . bacterial colonies grow lacy branches that all curl in the same direction , and human cells cultured inside a ring-shaped boundary tend to line up like the ridges on a cruller . if we zoom in even more , we see that many of cells ' basic building blocks , like nucleic acids , proteins , and sugars , are inherently asymmetric . proteins have complex asymmetric shapes , and those proteins control which way cells migrate and which way embryonic cilia twirl . these biomolecules have a property called chirality , which means that a molecule and its mirror image are n't identical . like your right and left hands , they look the same , but trying to put your right in your left glove proves they 're not . this asymmetry at the molecular level is reflected in asymmetric cells , asymmetric embryos , and finally asymmetric organisms . so while symmetry may be beautiful , asymmetry holds an allure of its own , found in its graceful whirls , its organized complexity , and its striking imperfections .

symmetry is everywhere in nature , and we usually associate it with beauty : a perfectly shaped leaf , or a butterfly with intricate patterns mirrored on each wing . but it turns out that asymmetry is pretty important , too , and more common than you might think , from crabs with one giant pincer claw to snail species whose shells ' always coil in the same direction .

can the cell be chiral in its structure and organization ?

every chicken was once an egg , every oak tree an acorn , every frog a tadpole . the patch of mold on that old piece of bread in the back of your fridge , not so long ago that was one , solitary cell . even you were once but a gleam in your parents ' eyes . all these organisms share the same basic goal : to perpetuate their own existence . all lifeforms that we 've discovered so far stay alive by using basically the same rules , materials , and machinery . imagine a factory full of robots . these robots have two missions : one , keep the factory running , and two , when the time is right , set up an entirely new factory . to do those things , they need assembly instructions , raw materials , plenty of energy , a few rules about when to work normally , when to work quickly , or when to stop , and some exchange currencies because even robots need to get paid . each factory has a high security office with blueprints for all the possible factory configurations and complete sets of instructions to make all the different types of robots a factory could ever need . special robots photocopy these instructions and send them off to help make the building blocks of more robots . their colleagues assemble those parts into still more robots , which are transported to the right location in the factory and given the tools they need to start working . every robot draws energy from the central power plant , a giant furnace that can burn regular fuel but also scrap materials if not enough regular fuel is available . certain zones in the factory have harsher working conditions , so these areas are walled off . but the robots inside can at least communicate with the rest of the factory through specialized portals embedded directly into the walls . and as you 've probably figured out , what we 're describing here is a cell . the high security office is the nucleus . it stores the blueprints and instructions as deoxyribonucleic acid , or dna . the photocopied instructions are rna . the robots themselves are mostly proteins built from amino acids , but they 'll often use special tools that are , or are derived from , vitamins and minerals . the walls between factory zones and around the factory itself are mostly made up of lipids , a.k.a . fats . in most organisms , the primary fuel source are sugars , but in a pinch , fats and proteins can be broken down and burned in the furnace as well . the portals are membrane proteins which allow very specific materials and information to pass through the walls at the right times . many interactions between robot proteins require some kind of push , think robot minimum wage . a few small but crucial forms of money are transferred between proteins to provide this push . electrons , protons , oxygen , and phosphate groups are the main chemical currencies , and they 're kept in small molecular wallets or larger tote bags to keep them safe . this is biochemistry , the study of how every part of the factory interacts to keep your life running smoothly in the face of extreme challenges . maybe there 's too much fuel ; your body will store the excess as glycogen or fat . maybe there 's not enough ; your body will use up those energy reserves . maybe a virus or bacteria tries to invade ; your body will mobilize the immune system . maybe you touched something hot or sharp ; your nerves will let you know so you can stop . maybe it 's time to create a new cell or a new person . amazingly , oak trees , chickens , frogs , and , yes , even you share so many of the same basic robot and factory designs that biochemists can learn a lot about all of them all at the same time .

all lifeforms that we 've discovered so far stay alive by using basically the same rules , materials , and machinery . imagine a factory full of robots . these robots have two missions : one , keep the factory running , and two , when the time is right , set up an entirely new factory . to do those things , they need assembly instructions , raw materials , plenty of energy , a few rules about when to work normally , when to work quickly , or when to stop , and some exchange currencies because even robots need to get paid .

in the very beginning of the video , we say that the robots have two missions : ( 1 ) to keep the factory going and ( 2 ) to set up a new factory . is there ever a time when these two missions might conflict with each other ? suggest a few scenarios when this might happen .

have you ever daydreamed about traveling through time , perhaps fast forward in the centuries and seeing the distant future ? well , time travel is possible , and what 's more , it 's already been done . meet sergei krikalev , the greatest time traveler in human history . this russian cosmonaut holds the record for the most amount of time spent orbiting our planet , a total of 803 days , 9 hours , and 39 minutes . during his stay in space , he time traveled into his own future by 0.02 seconds . traveling at 17,500 miles an hour , he experienced an effect known as time dilation , and one day the same effect might make significant time travel to the future commonplace . to see why moving faster through space affects passage of time , we need to go back to the 1880s , when two american scientists , albert michelson and edward morley , were trying to measure the effect of the earth 's movement around the sun on the speed of light . when a beam of light was moving in the same direction as the earth , they expected the light to travel faster . and when the earth was moving in the opposite direction , they expected it to go slower . but they found something very curious . the speed of light remained the same no matter what the earth was doing . two decades later , albert einstein was thinking about the consequences of that never-changing speed of light . and it was his conclusions , formulated in the theory of special relativity , that opened the door into the world of time travel . imagine a man named jack , standing in the middle of a train carriage , traveling at a steady speed . jack 's bored and starts bouncing a ball up and down . what would jill , standing on the platform , see through the window as the train whistles through ? well , between jack dropping the ball and catching it again , jill would have seen him move slightly further down the track , resulting in her seeing the ball follow a triangular path . this means jill sees the ball travel further than jack does in the same time period . and because speed is distance divided by time , jill actually sees the ball move faster . but what if jack 's bouncing ball is replaced with two mirrors which bounce a beam of light between them ? jack still sees the beam dropping down and jill still sees the light beam travel a longer distance , except this time jack and jill can not disagree on the speed because the speed of light remains the same no matter what . and if the speed is the same while the distance is different , this means the time taken will be different as well . thus , time must tick at different rates for people moving relative to each other . imagine that jack and jill have highly accurate watches that they synchronize before jack boards the train . during the experiment , jack and jill would each see their own watch ticking normally . but if they meet up again later to compare watches , less time would have elapsed on jack 's watch , balancing the fact that jill saw the light move further . this idea may sound crazy , but like any good scientific theory , it can be tested . in the 1970s , scientists boarded a plane with some super-accurate atomic clocks that were synchronized with some others left on the ground . after the plane had flown around the world , the clocks on board showed a different time from those left behind . of course , at the speed of trains and planes , the effect is minuscule . but the faster you go , the more time dilates . for astronauts orbiting the earth for 800 days , it starts to add up . but what affects humans also affects machines . satellites of the global positioning system are also hurdling around the earth at thousands of miles an hour . so , time dilation kicks in here , too . in fact , their speed causes the atomic clocks on board to disagree with clocks on the ground by seven millionths of a second daily . left uncorrected , this would cause gps to lose accuracy by a few kilometers each day . so , what does all this have to do with time travel to the far , distant future ? well , the faster you go , the greater the effect of time dilation . if you could travel really close to the speed of light , say 99.9999 % , on a round-trip through space for what seemed to you like ten years , you 'd actually return to earth around the year 9000. who knows what you 'd see when you returned ? ! humanity merged with machines , extinct due to climate change or asteroid impact , or inhabiting a permanent colony on mars . but the trouble is , getting heavy things like people , not to mention space ships , up to such speeds requires unimaginable amounts of energy . it already takes enormous particle accelerators like the large hadron collider to accelerate tiny subatomic particles to close to light speed . but one day , if we can develop the tools to accelerate ourselves to similar speeds , then we may regularly send time travelers into the future , bringing with them tales of a long , forgotten past .

well , time travel is possible , and what 's more , it 's already been done . meet sergei krikalev , the greatest time traveler in human history . this russian cosmonaut holds the record for the most amount of time spent orbiting our planet , a total of 803 days , 9 hours , and 39 minutes .

how many seconds did sergei krikalev time travel into his own future ?

in ancient greece , violent internal conflict between bordering neighbors and war with foreign invaders was a way of life , and greeks were considered premier warriors . most greek city-states surrounded themselves with massive defensive walls for added protection . sparta in its prime was a different story , finding walls unnecessary when it had an army of the most feared warriors in the ancient world . so what was sparta doing differently than everyone else to produce such fierce soldiers ? to answer that question , we turn to the written accounts of that time . there are no surviving written accounts from spartans themselves , as it was forbidden for spartans to keep records , so we have to rely on those of non-spartan ancient historians , like herodotus , thucydides , and plutarch . these stories may be embellished and depict sparta at the apex of its power , so take them with a grain of salt . for spartans , the purpose for their existence was simple : to serve sparta . on the day of their birth , elder spartan leaders examined every newborn . the strong healthy babies were considered capable of fulfilling this purpose , and the others may have been left on mount taygetus to die . every spartan , boy or girl , was expected to be physically strong , mentally sharp , and emotionally resilient . and it was their absolute duty to defend and promote sparta at all costs . so in the first years of their lives , children were raised to understand that their loyalty belonged first to sparta , and then to family . this mindset probably made it easier for the spartan boys , who upon turning seven , were sent to the agoge , a place with one main purpose : to turn a boy into a spartan warrior through thirteen years of relentless , harsh , and often brutal training . the spartans prized physical perfection above all else , and so the students spent a great deal of their time learning how to fight . to ensure resilience in battle , boys were encouraged to fight among themselves , and bullying , unlike today , was acceptable . in order to better prepare the boys for the conditions of war , the boys were poorly fed , sometimes even going days without eating . they also were given little in the way of clothing so that they could learn to deal with different temperatures . spartan boys were encouraged to steal in order to survive , but if they were caught , they would be disciplined , not because they stole , but because they were caught in the act . during the annual contest of endurance in a religious ritual known as the diamastigosis , teenage boys were whipped in front of an altar at the sanctuary of artemis orthia . it was common for boys to die on the altar of the goddess . fortunately , not everything was as brutal as that . young spartans were also taught how to read , write , and dance , which taught them graceful control of their movements and helped them in combat . while the responsibilities for the girls of sparta were different , the high standards of excellence and expectation to serve sparta with their lives remained the same . spartan girls lived at home with their mothers as they attended school . their curriculum included the arts , music , dance , reading , and writing . and to stay in peak physical condition , they learned a variety of sports , such as discus , javelin , and horseback riding . in sparta , it was believed that only strong and capable women could bear children that would one day become strong and capable warriors . to all spartans , men and women , perhaps the most important lesson from spartan school was allegiance to sparta . to die for their city-state was seen as the completion of one 's duty to sparta . upon their death , only men who died in battle and women who died in childbirth were given tombstones . in the eyes of their countrymen , both died so that sparta could live .

and to stay in peak physical condition , they learned a variety of sports , such as discus , javelin , and horseback riding . in sparta , it was believed that only strong and capable women could bear children that would one day become strong and capable warriors . to all spartans , men and women , perhaps the most important lesson from spartan school was allegiance to sparta . to die for their city-state was seen as the completion of one 's duty to sparta .

spartan boys were built to become the perfect warriors . what methods were used at “ spartan school ” to build them up both body and mind ? why is this mix important in building better warriors ?

`` some are born great , some achieve greatness , and others have greatness thrust upon them '' , quoth william shakespeare . or did he ? some people question whether shakespeare really wrote the works that bear his name , or whether he even existed at all . they speculate that shakespeare was a pseudonym for another writer , or a group of writers . proposed candidates for the real shakespeare include other famous playwrights , politicians and even some prominent women . could it be true that the greatest writer in the english language was as fictional as his plays ? most shakespeare scholars dismiss these theories based on historical and biographical evidence . but there is another way to test whether shakespeare 's famous lines were actually written by someone else . linguistics , the study of language , can tell us a great deal about the way we speak and write by examining syntax , grammar , semantics and vocabulary . and in the late 1800s , a polish philosopher named wincenty lutosławski formalized a method known as stylometry , applying this knowledge to investigate questions of literary authorship . so how does stylometry work ? the idea is that each writer 's style has certain characteristics that remain fairly uniform among individual works . examples of characteristics include average sentence length , the arrangement of words , and even the number of occurrences of a particular word . let 's look at use of the word thee and visualize it as a dimension , or axis . each of shakespeare 's works can be placed on that axis , like a data point , based on the number of occurrences of that word . in statistics , the tightness of these points gives us what is known as the variance , an expected range for our data . but , this is only a single characteristic in a very high-dimensional space . with a clustering tool called principal component analysis , we can reduce the multidimensional space into simple principal components that collectively measure the variance in shakespeare 's works . we can then test the works of our candidates against those principal components . for example , if enough works of francis bacon fall within the shakespearean variance , that would be pretty strong evidence that francis bacon and shakespeare are actually the same person . what did the results show ? well , the stylometrists who carried this out have concluded that shakespeare is none other than shakespeare . the bard is the bard . the pretender 's works just do n't match up with shakespeare 's signature style . however , our intrepid statisticians did find some compelling evidence of collaborations . for instance , one recent study concluded that shakespeare worked with playwright christopher marlowe on `` henry vi , '' parts one and two . shakespeare 's identity is only one of the many problems stylometry can resolve . it can help us determine when a work was written , whether an ancient text is a forgery , whether a student has committed plagiarism , or if that email you just received is of a high priority or spam . and does the timeless poetry of shakespeare 's lines just boil down to numbers and statistics ? not quite . stylometric analysis may reveal what makes shakespeare 's works structurally distinct , but it can not capture the beauty of the sentiments and emotions they express , or why they affect us the way they do . at least , not yet .

but there is another way to test whether shakespeare 's famous lines were actually written by someone else . linguistics , the study of language , can tell us a great deal about the way we speak and write by examining syntax , grammar , semantics and vocabulary . and in the late 1800s , a polish philosopher named wincenty lutosławski formalized a method known as stylometry , applying this knowledge to investigate questions of literary authorship .

what is the study of language ?

[ music ] science is coming . [ music ] the game of thrones universe is one of the most brilliantly complex and utterly frustrating fictional universes ever created . but it is a fictional universe , and the only rule of a fictional universe is that it is self-consistent . it does n't have to agree with our science , or logic , or even our commonly agreed-upon moral code that says killing people is not a good thing . there is only one god in that universe , and his name is gurm . but despite that , many things in game of thrones can be linked to the real , actual world , drawing inspiration as if through the thirsty roots of a weirwood tree . many of these connections are interpreted by fans , but some have been verified by the bearded one himself . there are the many competing religious philosophies , the many , many , many similarities to real-life historical characters or the fact that they look like us ? but we are not going to be talking about those ... here 's where i would give you a spoilers warning , but . . . come on . you clicked on this . spoilers are coming . why are the seasons so crazy ? in the game of thrones universe winters and summers are known to last years at a time and apparently show up when they damn well please . we know that the summer/winter cycle normally averages around 5 or 6 years apiece , and as the story begins the most recent summer has stretched to nearly ten years . on earth , seasons are caused by our axial tilt leaning one side of earth toward or away from the sun during our annual trip around it , but george 's world is n't so predictable . the maesters of the citadel are the geeks of westeros , who are supposed to calculate when the next polar doom will arrive . westeros is n't an industrial society , but the architecture , metallurgy , and medicine we see in the known world suggests that these guys are a fairly scientific bunch . many theories have attempted to explain the reason for these seasons , but most of them collapse faster than a greyjoy 's loyalty . we know that the westeros-ian world has a moon , and that it used to have two . maybe their moon is n't as large as ours so the planet 's axis , unstabilized by lunar gravity , wobbles like a broken top . but , according to astronomers , moons do n't stabilize planets , rather a moonless planet should spin more evenly than one with a moon . then what if its orbit , instead of a nearly circular ellipse , like ours , was extremely elongated ? well that does n't work either . while it could cause extreme seasons , they 'd still show up on a regular schedule . even complex combinations of orbital stretches and wobbles , like earth 's milankovitch cycles , could be predicted by any society that knows basic algebra . well , then maybe it 's tugged on by the gravity of a neighboring planet , or its sun has a variable output . george r.r . martin did write his first novel about a planet falling away from its parent star . it 's most likely that the game of thrones planet . . . it needs a name . planet hodor ! lives in a very strange solar system , around a pair of stars . last april , a group of graduate students from johns hopkins university published a paper showing that if the world of game of thrones was subject to the complex dynamics of three celestial bodies orbiting each other , predicting a planet 's seasons would be impossible . this has interesting implications for tatooine . . ? of course , it could also be due to magic , which is cheating . and what about that world anyway ? at the amazing planetary science blog generation anthropocene , miles traer and mike osborne have constructed a detailed geologic history of westeros stretching back more than 500 million years into the fictional past . they determined that , since the north is cold enough to maintain a wall of ice , which we 'll come back to , year-round , it must be near this planet 's arctic circle , and since the south is warm enough to be covered by deserts , which primarily exist near earth 's 30th parallel that planet hodor has a radius of 4,297 miles or about 10 % wider than earth . we know that the first men crossed into westeros on a land bridge near dorne , and like africa and south america , the coastlines of westeros and essos seem to fit like puzzle pieces . they were probably unzipped beginning 25 million years ago by a spreading rift , like the one in the middle of our atlantic ocean . and 40 million years ago , westeros was likely covered by a huge ice sheet , which retreated as glaciers , cutting the great valleys south of winterfell and the riverlands between harrenhal and the twins . the description of the jagged black mountains sounds a lot like our own rocky mountains , which were born around 60-80 million years ago . this would also mark the birth of the mountains of the moon and the high westerlands , as north and south westeros smashed together just like the fault beneath the himalayas . that violent uplift is what exposed all that lannister gold from its origin deep within the crust . that era also would have borne the iron islands . . . but we have a different iron to talk about . valyrian steel was an alloy forged in the ancient empire of the valyrians , lighter and stronger than regular steel , and whose secrets were lost during the great doom , when volcanoes torched valyria , and its dragons , into charcoal . that valyrian steel was forged with dragon fire , which is not actually a thing , but it 's almost certainly a reference to damscus steel , an ancient steel alloy developed in india around 300 bc . like valyrian steel , the secrets of its forgery were lost to history forever . speaking of dragon fire . . . what if dragons could exist ? how could a living thing breathe flames ? my buddy kyle hill came up with an interesting theory . like the tiny bombardier beetle , dragons could secrete reactive `` hypergolic '' chemicals that , when mixed , react violently and shoot out of an orifice like rocket fuel . and if dragons chewed on certain rocks and metals , which i imagine are like cupcakes to them , they could coat their teeth in minerals , creating a spark with rows of deadly knife-like flint and steel . unfortunately , our idea of a fire-breathing flying dragon is about to come crashing back down to earth , because physics . as bran stark found out the hard way , gravity seems to work in westeros just like it does here . and that means the mother of dragons ' kids are grounded . the largest bird that ever lived was the giant teratorn , with a wingspan of 7 meters . not big enough . dragons are probably more like pterosaurs . but even the largest of those , quetzalcoatlus , maxed out at 11 meters from wingtip to wingtip and 250 kg . but daenarys ' dragons are bigger than that by the time they hit puberty , and dragon lore says they never stop growing . even with a pterosaur 's hollow bones , ability to gallop on all fours to take off and huge stretchy wings , even hodor could figure out that the dragons do n't work . unless , yeah . . . magic . the wall ? wo n't work . a sheer cliff of solid ice stacked 700 feet tall would melt at the bottom under its own weight and would fall apart unless it was sloped wildfire ? works . `` greek fire '' was an ancient precursor to napalm made from petroleum , sulfur , saltpeter and was the most potent weapon of its time . add a little trimethyl borate , and you 've got a flaming death that 's ready for st. patrick 's day . milk of poppy ? works . our opiate drugs from morphine to vicodin to even heroin are all derived from the poppy plant . dire wolf ? works . the extinct canis dirus was the largest wolf to ever exist , covering north and south america , thousands have been found in the la brea tar pits alone . of course , the universe of game of thrones would live . . . or die . . . just fine whether or not it agrees with our science . but by combining the two , as raymond chandler said , the truth of art keeps science from becoming inhuman , and the truth of science keeps art from becoming ridiculous what do you think ? does bringing science into a fantasy story kill the wonder like a guest at the red wedding ? or does it help the fictional world . . . truly `` exist '' in our own ? i think it makes the story richer than a lannister . let me know what you think in the comments . and remember , a hanson always pay their debts . subscribe , and i will pay you back with a new video every week . valar morcurious .

martin did write his first novel about a planet falling away from its parent star . it 's most likely that the game of thrones planet . .

which of the following from game of thrones has a real earth equivalent ?

translator : tom carter reviewer : bedirhan cinar imagine a microscopic-sized ladder contained in the part of our brain that we 'll label our subconscious . the ladder of inference , which was first proposed by harvard professor chris argyris , is the basis of this model . every time we interact with someone , that experience enters the ladder at the bottom . that same experience zips up the ladder in the blink of an eye , exiting at the top . this process happens thousands of times a day without us knowing it . let 's focus on what happens on each rung of the ladder . on the first rung , we have the raw data and observations of our experience . this is very similar to what someone watching a video recording of our experience would see . moving up to the second rung , we filter in specific information and details from our experience . we unknowingly filter based on our preferences , tendencies , and many other aspects that we believe are important . on to the third rung . we assign meaning to the information we have filtered through . this is where we start to interpret what our information is telling us . on our fourth rung , a very crucial thing happens . we develop assumptions based on the meaning we created on the previous rung , and we start to blur the distinction between what is fact and what is story . on the fifth rung , we develop conclusions based on our assumptions . this is also where our emotional reactions are created . on the sixth rung , we adjust our beliefs about the world around us , including the person or people involved in our experience of the moment . on the seventh and final rung , we take action based on our adjusted beliefs . still with me ? great ! let 's take a real-life example and run it up the ladder to see how this all works . have you ever been cut off in a parking lot , signal light on as you steer toward your coveted spot , only to slam on your brakes at the last minute as someone pulls in front of you and steals your spot away ? imagine that experience and notice all of the data and observations landing on the first rung of your ladder . now let 's watch what we pay attention to on the second rung . who cares that it 's sunny out and the birds are chirping ? the 50 % off sign outside of your favorite store is meaningless . you filter in the sensation of your grip tightening on the wheel , you feel your blood pressure rise , you hear the squeal of your brakes , and you notice the expression on the face of the other driver as he pulls in front of you and quickly looks away . time for our third rung . ever since you were young , your parents taught you the importance of waiting in line and taking your turn . you live and die by the rule of first come , first served . and now this guy has just stolen your spot . what gives ? up to the fourth rung we go . watch closely as our assumptions take over and our story creates itself . `` that stupid jerk , did n't his parents teach him anything ? how could he not see my signal light ? he must never pay attention ! why does he think he 's more important than anyone else ? '' jumping quickly to the fifth rung , we conclude that this guy is heartless , inconsiderate , he needs to be taught a lesson and put in his place . we feel angry , frustrated , vindictive , justified . on our sixth rung , we adjust our beliefs based on the experience . `` that 's the last time i give in ! next time someone tries to cut me off , tires will be smoking on the pavement as i squeal past them into my spot . '' and finally our last rung : we take action . we back up , pull up behind his car , honk our horn , and roll down our window to scream a few choice words as well . now imagine , he walks over quickly , apologizing . his wife , who 's almost due with their first baby , called him from inside the mall to say she is in labor and needs to get to the hospital immediately . we 're momentarily shocked , apologize profusely , and wish him luck as he rushes toward the entrance . what just happened here ? what changed ? why is this so significant ? in our parking lot example , our beliefs were short-circuited by the ladder of the other individual . `` my wife is in labor , i need to get there quick , there 's a parking spot . whew ! oh , jeez , i cut someone off . i 'd better apologize quickly so they do n't think i 'm a jerk . '' but what if we were able to short-circuit our ladders ourselves ? proactively , by choice ? guess what ? we can ! let 's return to our unique human function of free will . next time you notice yourself reacting to your experience , pay focused attention to your ladder . ask yourself what beliefs are at play , where do they come from ? what data and observations did you filter in as a result of your beliefs , and why ? are your assumptions valid and supported by facts ? would a different set of assumptions create different feelings , and result in new and better conclusions and actions ? we all have our own unique ladder . be mindful of yours , and help others to see theirs .

why is this so significant ? in our parking lot example , our beliefs were short-circuited by the ladder of the other individual . `` my wife is in labor , i need to get there quick , there 's a parking spot .

in the story of the inconsiderate driver in the parking lot , the main point ( in terms of the ladder of inference ) is :

the naming of clouds on a cold december evening in 1802 , a nervous young man named luke howard stood before the assembled members of a london science club about to give a lecture that would change his life and go on to change humanity 's understanding of the skies . luke howard was a pharmacist by profession , but he was a meteorologist by inclination , having been obsessed by clouds and weather since childhood . as a school boy , he spent hours staring out of the classroom window , gazing at the passing clouds . like everyone else at the time , he had no idea how clouds formed , or how they stayed aloft . but he enjoyed observing their endless transformations . by his own admission , luke paid little attention to his lessons , but fortunately for the future of meteorology , he managed to pick up a good knowledge of latin . compared to the other natural sciences , meteorology , the study of weather , was a late developer , mainly because weather is elusive . you ca n't snap off a piece of rainbow or a section of cloud for convenient study . you can , of course , collect rain water in calibrated containers , but all you really end up with are buckets of water . understanding clouds required a different approach , which is where luke howard 's idea came in . his simple insight based on years of observation was that clouds have many individual shapes but they have few basic forms . in fact , all clouds belong to one of three principle types to which howard gave the names : cirrus , latin for tendril or hair , cumulus , heap or pile , and stratus , layer or sheet . but that was n't the clever part . clouds are constantly changing , merging , rising , falling , and spreading throughout the atmosphere , rarely maintaining the same shapes for more than a few minutes . any successful naming system had to accommodate this essential instability , as howard realized . so , in addition to the three main cloud types , he introduced a series of intermediate and compound types as a way of including the regular transitions that occur among clouds . a high , whispy cirrus cloud that descended and spread into a sheet was named cirrostratus , while groups of fluffy cumulus clouds that joined up and spread were named stratocumulus . howard identified seven cloud types , but these have since been expanded to ten , cloud nine being the towering cumulonimbus thunder cloud , which is probably why being on cloud nine means to be on top of the world . howard 's classification had an immediate international impact . the german poet and scientist j.w . von goethe wrote a series of poems in praise of howard 's clouds , which ended with the memorable lines , `` as clouds ascend , are folded , scatter , fall , let the world think of thee who taught it all , '' while percy shelley also wrote a poem `` the cloud , '' in which each of howard 's seven cloud types was characterized in turn . but perhaps the most impressive response to the naming of clouds was by the painter john constable , who spent two summers on hampstead heath painting clouds in the open air . once they had been named and classified , clouds became easier to understand as the visible signs of otherwise invisible atmospheric processes . clouds write a kind of journal on the sky that allows us to understand the circulating patterns of weather and climate . perhaps the most important breakthrough in understanding clouds was realizing that they are subject to the same physical laws as everything else on earth . clouds , for example , do not float , but fall slowly under the influence of gravity . some of them stay aloft due to upward convection from the sun-heated ground , but most are in a state of slow , balletic descent . `` clouds are the patron goddesses of idle fellows , '' as the greek dramatist aristophanes wrote in 420 b.c . and nephology , the study of clouds , remains a daydreamer 's science , aptly founded by a thoughtful young man whose favorite activity was staring out of the window at the sky .

some of them stay aloft due to upward convection from the sun-heated ground , but most are in a state of slow , balletic descent . `` clouds are the patron goddesses of idle fellows , '' as the greek dramatist aristophanes wrote in 420 b.c . and nephology , the study of clouds , remains a daydreamer 's science , aptly founded by a thoughtful young man whose favorite activity was staring out of the window at the sky .

the study of clouds is known as :

and you can see that the phosphorus has oxidized in the air and it ’ s taken some of the paper with it to generate a nice ‘ p ’ for phosphorus . so this is a sample of my favourite element , this is phosphorus . phosphorus is a very , very reactive element , especially this type of phosphorus which is white phosphorus . phosphorus itself , elemental phosphorus , exists as a number of different allotropes , so a bit like carbon with diamond and graphite , and here we have one of these allotropes ; it ’ s white phosphorus . it exists as discrete molecules of four phosphorus atoms connected like a pyramid , and those four phosphorus atoms are really strained , really reactive and they want to really react with oxygen as quick as they can . this is the sort that is used in phosphorus bombs . it burns very easily and gives a very intense white light out of it . so instantaneously you can see that fantastic oxidation reaction as the phosphorus reacts with the molecular oxygen in the gas inside the flask . you can put it out quite easily by putting water on it , provided that it hasn ’ t got too hot and the water will just evaporate . very reactive form the other allotropes , black and red phosphorus , are not as reactive and are much , much easier for us to handle . then there ’ s red phosphorus which is very much less reactive which is what is used in matches . it is used to make the heads of so-called ‘ non-safety ’ matches , which are the ones you can strike on any sort of material , on bricks and so on , and they tend to be this rather red colour here . so the phosphorus , you ’ ll notice , that it ’ s stored under water , and it ’ s stored under water to keep the oxygen and the atmosphere away because it reacts very vigorously and can burn , hence the signs , it ’ s very , very flammable . it ’ s so flammable in fact that it burns just with the heat of the skin . so you can see a number of scars across my hands , these are all from playing with phosphorus and doing reactions and experiments with phosphorus . phosphorus is a very important point inside your body . some of you may have read aldous huxley ’ s book called ‘ brave new world ’ , which when people died they were sent to the phosphorus reclamation plant . and each average size person contains somewhat over a pound of phosphorus inside their bodies , and nowadays people can use magnetic resonance imaging to look and see how the phosphorus is used inside the body . so i ’ ve taken a small sample of my white phosphorus and i ’ ve dissolved it in an organic solvent which is carbon disulphide , because the p4 is molecular and it dissolves really quite nicely in this molecular solvent . so now what we are going to do is we ’ re going to go outside and we ’ re going to put this on a piece of filter paper and then let the carbon disulphide evaporate so we can see the reaction of the phosphorus with the oxygen and the paper . and the average person produces inside their body about 70 kilos a day of a molecule called atp , adenosine triphosphate , which is the energy producing molecule inside the body . now you don ’ t keep 70 kilos inside you , or you would explode , but you ’ re using it up all the time but in the course of the day you synthesise nearly 70 kilos of this molecule . so as the carbon disulphide evaporates from the paper , well there ’ s a couple of drips . and you can see that the phosphorus has oxidized in the air and it ’ s taken some of the paper with it to generate a nice ‘ p ’ for phosphorus .

so now what we are going to do is we ’ re going to go outside and we ’ re going to put this on a piece of filter paper and then let the carbon disulphide evaporate so we can see the reaction of the phosphorus with the oxygen and the paper . and the average person produces inside their body about 70 kilos a day of a molecule called atp , adenosine triphosphate , which is the energy producing molecule inside the body . now you don ’ t keep 70 kilos inside you , or you would explode , but you ’ re using it up all the time but in the course of the day you synthesise nearly 70 kilos of this molecule .

phosphorus is essential for life . phosphate is a component of dna and also of an energy-producing molecule in your body . what is name of this molecule ?

hi , i ’ m john green . this is crash course world history and today we ’ re going to talk about world war ii . finally , a war with some color film ! so , here at crash course we try to make history reasonably entertaining , and fortunately , world war ii was hilarious ... said no one ever . mr. green , mr. green ! is this , like , gon na be one of the unfunny ones where you build to the big melodramatic conclusion about how i have to imagine the world more complexly ? me from the past , as long as you have that eighth rate soup-strainer , i ’ m not even going to acknowledge your existence . [ theme music ] right , so you ’ ve probably heard a lot about world war ii from movies and books , the history channel , before it decided that swamp people were history , the incessant droning of your grandparents , etc . we ’ re not gon na try to give you a detailed synopsis of the war today . instead , we ’ re going to try to give a bit of perspective on how the most destructive war in human history happened , and why it still matters globally . so one of the reasons history classes tend to be really into wars is that they ’ re easy to put on tests . they start on one day and they end on another day . and they ’ re caused by social , political , and economic conditions that can be examined in a multiple choice kind of manner . except , not really . like , when did world war ii start ? in september 1939 , when the nazis invaded poland ? i ’ d say no - it actually started when japan invaded manchuria in 1931 , or at the very latest when the japanese invaded china in 1937 , because they didn ’ t stop fighting until 1945 . then again , you could also argue 1933 , when hitler took power , or 1941 , when america started fighting . it ’ s complicated . but anyway , in china the fighting was very brutal , as exemplified by the infamous rape of nanking , which featured the slaughter of hundreds of thousands of chinese people and is still so controversial today that : 1 . it affects relations between japan & amp ; amp ; china and 2 . even though i have not described it in detail , you can rest assured that there will be angry comments about my use of the word “ slaughter. ” but the world war ii we know the most about from movies and tv is primarily the war in the european theater , the one that adolf hitler started . hitler is the rare individual who really did make history - specifically he made it worse - and if he hadn ’ t existed , it ’ s very unlikely that world war ii would ’ ve ever happened . but he did exist , and after coming to power in 1933 , with the standard revolutionary promises to return the homeland to its former glory , infused with quite a bit of paranoia and anti-semitism , germany saw rapid re-militarization and eventually , inevitably , war . in the beginning , it was characterized by a new style of combat made possible by the mechanized technology of tanks , airplanes , and especially , trucks . this was the blitzkrieg , a devastating tactic combining quick movement of troops , tanks , and massive use of air power to support infantry movements . and in the very early years of the war , it was extremely effective . the nazis were able to roll over poland , norway , denmark , the netherlands , and then all of france , all within about 9 months between the fall of 1939 and the summer of 1940 . so after knocking out most of central europe , the nazis set their sights on great britain , but they didn ’ t invade the island , choosing instead to attack it with massive air strikes . i mean , you look at this poster and think , “ man , the queen wants me to finish my term paper , so i can do it , ” but when this poster was first produced in 1939 , it was to quell terror in the face of bombardment . the battle of britain was a duel between the royal air force and the luftwaffe , and while the raf denied the nazis total control of british airspace , the nazis were still able to bomb great britain over and over again in what ’ s known as the blitz . stan , no . no jokes this time . yes , the blitz . meanwhile , europeans were also fighting each other in north africa . the desert campaigns started in 1940 and lasted through 1942 - this is where british general “ monty ” montgomery outfoxed german general irwin “ the desert fox ” rommel . it ’ s also the place where americans first fought nazis in large numbers . but most importantly , it ’ s where indiana jones discovered the ark of the covenant . okay , let ’ s go to the thought bubble . 1941 was a big year for world war ii . first , the nazis invaded russia , breaking a non-aggression pact that the two powers had signed in 1939 . this hugely escalated the war , and also made allies of the most powerful capitalist countries and the most powerful communist one , an alliance that would stand the test of time and never end ... until like three seconds after the defeat of the nazis . the nazi invasion of russia opened the war up on the so-called eastern front , although if you were russian , it was the western front , and it led to millions of deaths , mostly russian . also , 1941 saw a day that would `` live in infamy '' when the japanese bombed pearl harbor , hoping that such an audacious attack would frighten the united states into staying neutral , which was a pretty stupid gamble because : 1 . the u.s. was already giving massive aid to the allies and was hardly neutral and 2 . the united states is not exactly famed for its pacifism or political neutrality . 1941 also saw japan invading much of southeast asia , which made australia and new zealand understandably nervous . as part of the british commonwealth , they were already involved in the war , but now they could fight the japanese closer to home . and shut up about how i never talk about you australians . i just gave you 1.5 sentences . but by the time the americans and australians started fighting the japanese , it was already a world war . sometimes this meant fighting or starving or being bombed ; other times , it meant production for the war - you don ’ t think of argentina as being a world war ii powerhouse , for instance , but they were vital to the allies , supplying 40 % of british meat during world war ii . thanks , thought bubble . so , not to sound jingoistic , but the entry of the u.s. into the war really did change everything , although i doubt the nazis could ’ ve taken russia regardless . no one conquers russia in the wintertime , unless you are - wait for it - the mongols . okay , we ’ re going to skip most of the big battles of 1942 - like the battle of midway , which effectively ended japan ’ s chance of winning the war - and focus on the battle of stalingrad . the german attack on stalingrad , now known as volgograd because stalin sucks , was one of the bloodiest battles in the history of war , with more than two million dead . the germans began by dropping more than 1,000 tons of bombs on stalingrad , and then the russians responded by “ hugging ” the germans , staying as close to their front lines as possible so that german air support would kill germans and russians alike . this kind of worked , although the germans still took most of the city . but then , a soviet counterattack left the sixth army of the nazis completely cut off . and after that , due partly to hitler ’ s overreaching megalomania and partly to lots of people being scared of him , the sixth army slowly froze and starved to death before finally surrendering . and of the 91,000 axis pows from stalingrad , only about 6,000 ever returned home . stalingrad turned the war in europe and by 1944 , the american strategy of “ island hopping ” in the pacific was taking gis closer and closer to japan . rome was liberated in june by americans and canadians ; and the successful british , canadian , and american d-day invasion of normandy was the beginning of the end for the nazis . oh , it ’ s time for the open letter ? an open letter to canada . but first , let ’ s see what ’ s in the secret compartment today . oh , it ’ s canadian mittens . i wan na thank the canadian crash course fans , who sent us these mittens . canadians are just so nice , stan . like , all we ever do on this show is make fun of them , and they ’ re just like , “ it ’ s so kind of you to mention us . here ’ s some mittens ! ” dear canada , we ’ re not always nice to you here on crash course , but you are awesome . i ’ m pointing , but you can ’ t tell because i ’ m wearing mittens . 45,000 canadians died fighting for the allies in world war ii , which means that , per capita , canada lost more people than the united states . you fought with the royal air force to defend great britain from the beginning of the war and you were there on d-day , successfully invading juno beach . and , as many of you have pointed out in comments , you defeated the united states in the war of 1812 , meaning that , arguably , canada , you are the greater military power . plus , you have lumberjacks , and excellent beer , and hockey , and universal healthcare , and justin bieber . i ’ m jealous ! that 's what it is - i 'm jealous ! best wishes , john green . so , by the end of 1944 , the allies were advancing from the west and the russian red army was advancing from the east and then , the last-ditch german offensive at the battle of the bulge in the winter of 1944-1945 failed . mussolini was executed in april of 1945 . hitler committed suicide at the end of that month . and , on may 8 , 1945 the allies declared victory in europe after germany surrendered unconditionally . three months later , the united states dropped the only two nuclear weapons ever deployed in war , japan surrendered , and world war ii was over . the war had a definite cause : unbridled military expansion by germany , japan , and , to a small extent , italy . now , it ’ s easy to claim that hitler was crazy or evil , and , in fact , he was certainly both , but that doesn ’ t explain the nazis decision to invade russia , and it sure doesn ’ t explain japan ’ s decision to bomb pearl harbor . and there are many possible explanations beyond mere evil ; but the most interesting one , to me , involves food . hitler had a number of reasons for wanting to expand germany ’ s territory , but he often talked about lebensraum or living space for the german people . german agriculture was really inefficiently organized into lots of small farms , and that meant that germany needed a lot of land in order to be self-sufficient in food production . the plan was to take poland , the ukraine , and eastern russia , and then resettle that land with lots of germans , so that it could feed german people . this was called the hunger plan because the plan called for 20 million people to starve to death . many would be the poles , ukrainians , and russians who ’ d previously lived on the land . the rest would be europe ’ s jews , who would be worked to death . six million jews were killed by the nazis , many by starvation , but many through a chillingly planned effort of extermination in death camps . these death camps can be distinguished from concentration camps or labor camps in that their primary purpose was extermination of jews , roma people , communists , homosexuals , disabled people , and others that the nazis deemed unfit . some historians believe that the nazis opened the death camps because the jews weren ’ t dying as fast as the hunger plan had intended . this was a sickening plan , but it made a kind of demented sense . rather than becoming more involved in global trade , as the british had , the germans would feed themselves by taking land and killing the people who ’ d previously lived there . similarly , japan , at the beginning of the war , was suffering from an acute fear of food shortage because its agricultural sector was having trouble keeping up with population growth . and the japanese too , sought to expand their agricultural holdings by , for instance , resettling farmers in korea . so while it ’ s tempting to say that world war ii was about the allies fighting for democratic ideals against the totalitarian militaristic imperialism of the fascist axis powers , it just doesn ’ t hold up to scrutiny . for instance , a hugely important allied power , stalin ’ s soviet union , was , like , the least democratic place , ever . stan just said that was hyperbole , but it ’ s not . stalin ’ s soviet union is tied with all of the other completely undemocratic countries for last place on the democracy scale . it ’ s a big community there , at last place , but they ’ re definitely in there somewhere . and , by far , the biggest imperialists of the war were the british . they couldn ’ t have fed or clothed themselves - or resisted the nazis - without their colonies and commonwealth . so , why is world war ii so important ? well first , it proved the old roman adage homo homini lupus : man is a wolf to man . this is seen most clearly in the holocaust , but all the statistics are staggering . more than a million indian british subjects died , mainly due to famine that could have been avoided if the british had redistributed food . and their failure to do so helped convince indians that the so-called superior civilization of the british was a sham . more than a million vietnamese died , mainly due to famine . 418,000 americans . more than a million noncombatants in both germany and japan . and 20 million people in the soviet union , most of them civilians . these civilians were targeted because they helped sustain the war , mostly through industrial and agricultural production . in a total war , when a nation is at war , not just its army , there is no such thing as a non-military target . from the firebombing of dresden to tokyo to hiroshima , the line between soldier and civilian blurred . and then , of course , there is the holocaust , which horrifies us because the elements of western progress - record-keeping , industrial production , technology - were used to slaughter millions . world war ii saw modern industrial nations , which represented the best of the enlightenment and the scientific revolution , descend into once unimaginable cruelty . and what makes world war ii such a historical watershed is that in its wake , all of us - in the west or otherwise - were forced to question whether western dominance of this planet could , or should , be considered progress . thanks for watching . i ’ ll see you next week . crash course is produced and directed by stan muller . our script supervisor is meredith danko . our associate producer is danica johnson . the show is written by my high school history teacher , raoul meyer , and myself . and our graphics team is thought bubble . last week ’ s phrase of the week was “ an end to history. ” if you want to guess at this week ’ s phrase of the week or suggest future ones , you can do so in comments , where you can also ask questions about today ’ s video that will be answered by our team of historians . if you enjoy crash course , make sure you ’ re subscribed . thanks for watching , and as we say in my hometown , don ’ t forget to be awesome .

more than a million noncombatants in both germany and japan . and 20 million people in the soviet union , most of them civilians . these civilians were targeted because they helped sustain the war , mostly through industrial and agricultural production .

which country saw the deaths of 20 million of its citizens , most of whom were civilians ?

cell membranes are structures of contradictions . these oily films are hundreds of times thinner than a strand of spider silk , yet strong enough to protect the delicate contents of life : the cell 's watery cytoplasm , genetic material , organelles , and all the molecules it needs to survive . how does the membrane work , and where does that strength come from ? first of all , it 's tempting to think of a cell membrane like the tight skin of a balloon , but it 's actually something much more complex . in reality , it 's constantly in flux , shifting components back and forth to help the cell take in food , remove waste , let specific molecules in and out , communicate with other cells , gather information about the environment , and repair itself . the cell membrane gets this resilience , flexibility , and functionality by combining a variety of floating components in what biologists call a fluid mosaic . the primary component of the fluid mosaic is a simple molecule called a phospholipid . a phospholipid has a polar , electrically-charged head , which attracts water , and a non-polar tail , which repels it . they pair up tail-to-tail in a two layer sheet just five to ten nanometers thick that extends all around the cell . the heads point in towards the cytoplasm and out towards the watery fluid external to the cell with the lipid tails sandwiched in between . this bilayer , which at body temperature has the consistency of vegetable oil , is studded with other types of molecules , including proteins , carbohydrates , and cholesterol . cholesterol keeps the membrane at the right fluidity . it also helps regulate communication between cells . sometimes , cells talk to each other by releasing and capturing chemicals and proteins . the release of proteins is easy , but the capture of them is more complicated . that happens through a process called endocytosis in which sections of the membrane engulf substances and transport them into the cell as vesicles . once the contents have been released , the vesicles are recycled and returned to the cell membrane . the most complex components of the fluid mosaic are proteins . one of their key jobs is to make sure that the right molecules get in and out of the cell . non-polar molecules , like oxygen , carbon dioxide , and certain vitamins can cross the phospholipid bilayer easily . but polar and charged molecules ca n't make it through the fatty inner layer . transmembrane proteins stretch across the bilayer to create channels that allow specific molecules through , like sodium and potassium ions . peripheral proteins floating in the inner face of the bilayer help anchor the membrane to the cell 's interior scaffolding . other proteins in cell membranes can help fuse two different bilayers . that can work to our benefit , like when a sperm fertilizes an egg , but also harm us , as it does when a virus enters a cell . and some proteins move within the fluid mosaic , coming together to form complexes that carry out specific jobs . for instance , one complex might activate cells in our immune system , then move apart when the job is done . cell membranes are also the site of an ongoing war between us and all the things that want to infect us . in fact , some of the most toxic substances we know of are membrane-breaching proteins made by infectious bacteria . these pore-forming toxins poke giant holes in our cell membranes , causing a cell 's contents to leak out . scientists are working on developing ways to defend against them , like using a nano-sponge that saves our cells by soaking up the membrane-damaging toxins . the fluid mosaic is what makes all the functions of life possible . without a cell membrane , there could be no cells , and without cells , there would be no bacteria , no parasites , no fungi , no animals , and no us .

cell membranes are structures of contradictions . these oily films are hundreds of times thinner than a strand of spider silk , yet strong enough to protect the delicate contents of life : the cell 's watery cytoplasm , genetic material , organelles , and all the molecules it needs to survive .

cell membranes are ________ permeable .

what lights up the screen that you 're looking at right now ? trace back the battery chargers and power cords and you 'll end up at an electrical outlet , providing easy , safe access to reliable electricity . but beyond that outlet , the picture gets messier . it takes a lot of fuel to heat our homes , preserve our food , and our power our gadgets around the clock . and for 40 % of the world , that fuel is cheap , plentiful , and it 's called coal . but coal also releases pollutants into the air , like sulfur dioxide , nitrogen oxides , soot , and toxic metals , like mercury . these cause environmental damage , like acid rain , and serious health problems . in fact , in 1952 , coal burning caused such heavy smog in london that pedestrians could n't even see their feet , and thousands of people died from ill health . since then , many countries have deployed technology to remove most of these pollutants before they reach the air . but now we have a new air pollution problem on our hands , one that does n't show up in a cloud of dark smog , but in rising seas , floods , and heat waves . it 's global climate change , and again , the main culprit is coal . it 's responsible for 44 % of global carbon dioxide emissions , which trap the sun 's heat in the earth 's atmosphere , instead of letting it escape . so now the question is how do we remove that bad stuff as well ? that 's the idea behind cleaner coal . creating cleaner coal is really about trying to contain its ill effects with the help of special technologies that make the end product more acceptable . just like the most intriguing superheroes often have their own dark powers to overcome , so we can try and keep coal 's negative forces in check . but why do n't we just exterminate coal if it takes that much effort to clean it up ? simply , coal is extremely valuable to us , and it 's easy to come by . compressed underground for ages , coal holds chemical energy from plants that were fed from by the sun hundreds of millions of years ago , long before humans evolved . that makes coal energy dense , meaning it can be burned 'round the clock . it 's also cheap , if you ignore the pollution costs , and should last us through the end of the 21st century . we 've already got all the infrastructure in place for harnessing its power , and globally , although countries are making a move towards energy from cleaner and more renewable sources , there 's no sign yet that coal use is slowing down . in fact , as of 2012 , over 1000 new coal plants have been proposed , mostly in china and india . since for the time being coal is here to stay , experts say that if we want to reduce its emissions ' impact on the atmosphere , and slow down climate change , we 'll have to think of creative ways of reducing coal 's destructive power . to do that , we need to strip it of its foul forces , all that toxic carbon dioxide that causes havoc in the atmosphere . then , we need to store the co2 somewhere else . this mission is called carbon capture and sequestration , or ccs . and as if carbon dioxide were some evil genie we did n't want to escape , once it has been separated from coal , we 've devised ways to banish it underground . we can do this by injecting it deep into the earth , or by placing it deep under the ocean 's surface . stripping away coal 's negative elements can happen in three ways . first , and most commonly , as coal burns , the exhaust gas can be mixed with a compound called monoethanolamine . like a forceful power-stripping magnet , this compound bonds to the co2 , yanking it out of the gas stream so it can be stored separately underground . another method is to relieve coal of its co2 before it even has a chance to be released as exhaust . in this process , steam and oxygen swoop in to the rescue to convert coal into a special product called syngas , made up of carbon monoxide and hydrogen and some co2 . zap that with some water vapor , and the carbon monoxide gets converted into carbon dioxide , which can be isolated . the leftover hydrogen gas is then used as energy to generate electricity , so there 's an added bonus . a third technique exposes coal to pure oxygen , instead of burning it in air . this creates exhaust gas with higher concentrations of carbon dioxide , which makes it easy to isolate and to banish to the chasms below . all this can reduce emissions at a power plant by up to 90 % , but as with any superhero struggling with their destructive powers , it takes a lot of effort to switch over from the dark side . so these positive pollution-busting forces , although they 're available , have barely been used in commercial power plants because they cost a lot . but ultimately , the bigger problem is that in most parts of the world , it 's still too easy and much cheaper to keep emitting carbon dioxide , and that makes it tempting to completely ignore coal 's dark side . in this case , the most powerful force for good is regulation , the rules that can restrict the amount of carbon dioxide emitted from power plants , and make energy companies around the world wary of what they put into the air . until then , every time you turn on a screen or flick a light switch , coal is lurking in the background , carrying its dark powers with it wherever it goes .

so now the question is how do we remove that bad stuff as well ? that 's the idea behind cleaner coal . creating cleaner coal is really about trying to contain its ill effects with the help of special technologies that make the end product more acceptable .

which of these is a pollutant that coal emits when we burn it ?

in 2011 , a team of physicists reported a startling discovery : neutrinos traveled faster than the speed of light by 60 billionths of a second in their 730 kilometer trip from geneva to a detector in italy . despite six months of double checking , the bizarre discovery refused to yield . but rather than celebrating a physics revolution , the researchers published a cautious paper arguing for continued research in an effort to explain the observed anomaly . in time , the error was tracked to a single incorrectly connected fiber optic cable . this example reminds us that real science is more than static textbooks . instead , researchers around the world are continuously publishing their latest discoveries with each paper adding to the scientific conversation . published studies can motivate future research , inspire new products , and inform government policy . so it 's important that we have confidence in the published results . if their conclusions are wrong , we risk time , resources , and even our health in the pursuit of false leads . when findings are significant , they are frequently double-checked by other researchers , either by reanalyzing the data or by redoing the entire experiment . for example , it took repeated investigation of the cern data before the timing error was tracked down . unfortunately , there are currently neither the resources nor professional incentives to double check the more than 1 million scientific papers published annually . even when papers are challenged , the results are not reassuring . recent studies that examined dozens of published pharmaceutical papers managed to replicate the results of less than 25 % of them . and similar results have been found in other scientific disciplines . there are a variety of sources for irreproducible results . errors could hide in their original design , execution , or analysis of the data . unknown factors , such as patients ' undisclosed condition in a medical study , can produce results that are not repeatable in new test subjects . and sometimes , the second research group ca n't reproduce the original results simply because they do n't know exactly what the original group did . however , some problems might stem from systematic decisions in how we do science . researchers , the institutions that employ them , and the scientific journals that publish findings are expected to produce big results frequently . important papers can advance careers , generate media interest , and secure essential funding , so there 's slim motivation for researchers to challenge their own exciting results . in addition , little incentive exists to publish results unsupportive of the expected hypothesis . that results in a deluge of agreement between what was expected and what was found . in rare occasions , this can even lead to deliberate fabrication , such as in 2013 , when a researcher spiked rabbit blood with human blood to give false evidence that his hiv vaccine was working . the publish or perish mindset can also compromise academic journals ' traditional peer-review processes which are safety checks where experts examine submitted papers for potential shortcomings . the current system , which might involve only one or two reviewers , can be woefully ineffective . that was demonstrated in a 1998 study where eight weaknesses were deliberately inserted into papers , but only around 25 % were caught upon review . many scientists are working toward improving reproducibility in their fields . there 's a push to make researchers raw data , experimental procedures , and analytical techniques more openly available in order to ease replication efforts . the peer review process can also be strengthened to more efficiently weed out weak papers prior to publication . and we could temper the pressure to find big results by publishing more papers that fail to confirm the original hypothesis , an event that happens far more than current scientific literature suggests . science always has , and always will , encounter some false starts as part of the collective acquisition of new knowledge . finding ways to improve the reproducibility of our results can help us weed out those false starts more effectively , keeping us moving steadily toward exciting new discoveries .

in rare occasions , this can even lead to deliberate fabrication , such as in 2013 , when a researcher spiked rabbit blood with human blood to give false evidence that his hiv vaccine was working . the publish or perish mindset can also compromise academic journals ' traditional peer-review processes which are safety checks where experts examine submitted papers for potential shortcomings . the current system , which might involve only one or two reviewers , can be woefully ineffective .

what does the phrase “ publish or perish ” refer to ?

there 's a common misconception that if you like to meticulously organize your things , keep your hands clean , or plan out your weekend to the last detail , you might have ocd . in fact , ocd , which stands for obsessive compulsive disorder , is a serious psychiatric condition that is frequently misunderstood by society and mental health professionals alike . so let 's start by debunking some myths . myth one : repetitive or ritualistic behaviors are synonymous with ocd . as its name suggests , obsessive compulsive disorder has two aspects : the intrusive thoughts , images , or impulses , known as obsessions , and the behavioral compulsions people engage in to relieve the anxiety the obsessions cause . the kinds of actions that people often associate with ocd , like excessive hand washing , or checking things repeatedly , may be examples of obsessive or compulsive tendencies that many of us exhibit from time to time . but the actual disorder is far more rare and can be quite debilitating . people affected have little or no control over their obsessive thoughts and compulsive behaviors , which tend to be time consuming and interfere with work , school or social life to the point of causing significant distress . this set of diagnostic criteria is what separates people suffering from ocd from those who may just be a bit more meticulous or hygiene obsessed than usual . myth two : the main symptom of ocd is excessive hand washing . although hand washing is the most common image of ocd in popular culture , obsessions and compulsions can take many different forms . obsessions can manifest as fears of contamination and illness , worries about harming others , or preoccupations with numbers , patterns , morality , or sexual identity . and compulsions can range from excessive cleaning or double checking , to the fastidious arrangement of objects , or walking in predetermined patterns . myth three : individuals with ocd do n't understand that they are acting irrationally . many individuals with ocd actually understand the relationship between their obsessions and compulsions quite well . being unable to avoid these thoughts and actions despite being aware of their irrationality is part of the reason why ocd is so distressing . ocd sufferers report feeling crazy for experiencing anxiety based on irrational thoughts and finding it difficult to control their responses . so what exactly causes ocd ? the frustrating answer is we do n't really know . however , we have some important clues . ocd is considered a neurobiological disorder . in other words , research suggests that ocd sufferers brains are actually hardwired to behave in a certain fashion . research has implicated three regions of the brain variously involved in social behavior and complex cognitive planning , voluntary movement , and emotional and motivational responses . the other piece of the puzzle is that ocd is associated with low levels of serotonin , a neurotransmitter that communicates between brain structures and helps regulate vital processes , such as mood , aggression , impulse control , sleep , appetite , body temperature and pain . but are serotonin and activity in these brain regions the sources of ocd or symptoms of an unknown underlying cause of the disorder . we probably wo n't know until we have a much more intimate understanding of the brain . the good news is there are effective treatments for ocd , including medications , which increase serotonin in the brain by limiting its reabsorption by brain cells , behavioral therapy that gradually desensitizes patients to their anxieties , and in some cases , electroconvulsive therapy , or surgery , when ocd does n't respond to other forms of treatment . knowing that your own brain is lying to you while not being able to resist its commands can be agonizing . but with knowledge and understanding comes the power to seek help , and future research into the brain may finally provide the answers we 're looking for .

so what exactly causes ocd ? the frustrating answer is we do n't really know . however , we have some important clues .

fear of contracting an illness such as hiv from touching public surfaces is referred to as a ( n ) _____ whereas frequently washing your hands in scalding hot water after touching such surfaces is referred to as a ( n ) _____ ?

translator : andrea mcdonough reviewer : bedirhan cinar so how many of you have a robot at home ? ok , i see about 20 , 30 hands . that 's actually pretty good . how many of you would want your own personal robot at home ? i know i would ! ok , so why does n't this exist ? why ca n't i go to the convenience store or the department store and , you know , go up to the cashier and say , `` yeah , i want my personal robot '' ? well , i 'm going to talk to you about how to make that happen . the thing that we need to do is to make robots smarter . now , no one will argue that we do n't have robots . we have rovers that are going to mars and are getting science data and expanding our understanding of the world . we have manufacturing robots that are helping to build our cars that we drive today . we even have robots that are helping our military , that are out disposing of bombs so our soldiers can come home safely . so we have all this , so why do n't we have the personal robot ? why do n't i have my robot chef ? because i ca n't cook . ( laughter ) so , here 's one of my robots , this is a simple walking robot , but it is by no means smart . and so , what we need to do is we need to change the definition of what a robot is . how do we do that ? well , the first step , before we even start designing and getting our hands dirty , we have to come up with rules , kind of the laws , rules of conduct . and why is this ? because if these robots are smart , they might be capable of more than we want . and so we have to come up with rules . thou , robot , shall not harm a human . thou shall obey me , and only me . thou shall always protect me at all possible times . so we have to lay the boundaries , the rules of engagement , before we actually start designing . and then we have to come up with tools . so i believe that the way to make robots smarter is to mimic people . now , our brains are complex , there 's a lot going on in there , and so , it 'd be hard to try to open up the brain and actually figure out how to mimic humans . the best way is to observe , is to actually watch people do things , and figure out what are they doing , what are their thoughts , what are their actions , what are their emotions ? and so , part of making robots smarter is actually trying to mimic humans , mimic how we do things , so maybe they can do it a little bit better . and so , some of the tools are varied . and so , i 'm classically trained as an electrical engineer . i never thought i 'd have to understand things like child psychology , infant development . so , understanding that the way infants develop to children , develop to adults , and how they learn and interact is actually important for robotics . i did n't understand that i 'd actually have to watch tapes of monkeys interacting and communicating , because they have a whole social kind of mechanism where they learn from each other , and so that 's really good to make robots smarter . and , of course , neuroscience , i 've always been fascinated with neuroscience , but i never understood that i had to figure out why do the neurons fire , what about the environment helps us to learn , and all of those really contribute to making robots a little bit smarter . and so , some of the things that i do -- and this is just a little snapshot -- one of the things is mirroring . so they say our ability to look in a mirror and wave and actually recognize that the person on the other side is us , that self-awareness , is a sign of intelligence , and that allows us to then look at someone pitch a ball and figure out , `` ok , i know how to pitch a ball , i 'm going to mirror their improvement . '' and so i actually have a robot where we are trying to design a robot health coach . and so , i have an exercise physiologist showing the robot how to do some exercises . you know , we want to get strong . and then , the other thing is learning . so , learning is important . we do this as children , we do this even as adults , we do this as elder . and yet , one form of learning is muscle memory . so how many of you play an instrument ? ok , so when you start off , for example , if you think about the violin , you start off and your instructor might actually come and move your hand a little bit or maybe move your bow a little bit up . so they actually touch you in order to give you muscle memory . and that helps you understand how to do things a little better . and so we actually have a learning methodology where of course , we 're not going to take the motors and move the legs , and so we have to nunchuk to give our robot muscle memory in terms of how to do dance moves . and then , lastly , is creativity . so , you might ask , `` robots ? creativity ? i do n't get this . why does the robot have to be creative ? what about creativity makes them smarter ? '' well , creativity and imagination , those are the things that allow us to create problems when we do n't know how to attack it . they allow us to make something out of nothing . i mean , if you look at the apps that at out there and the tablets , and the ipads , and the iphones , and the androids -- 20 years ago they did n't exist . so , how is it that we got from something where there was nothing and expanded ? it was our imagination . it was our creativity . and these are the things that allow us to figure out new things . and so , i have a robot that is creative , it plays piano , is a composer , and if you listen , it plays `` twinkle , twinkle little star . '' ( music ) so , all of this together , the last thing is interaction . so , you have a robot , you want it to be your playmate , your teacher , your instructor , you want it to interact . and is n't it so cute ? ( laughter ) so , interaction is key , it is key to understanding how to work in our world with us , and so the interaction piece is very important . it deals with communication , it deals with understanding , it deals with gaze , it deals with attention . all of these things together allow that interaction and our robots to be smart . and so these are just some of the tools that we use in order to make robots smarter . so , i want to leave you with one thought . so , i 'm all for robots and smart robots . i mean , that 's what i do , i 'd be out of a job if i did n't believe in that . but yet , where does it end ? how far do we push it ? how far and how smart should we make our smart robots ? thank you . ( applause )

and so these are just some of the tools that we use in order to make robots smarter . so , i want to leave you with one thought . so , i 'm all for robots and smart robots .

one thing that makes howard 's job as a roboticist so appealing is that she deals only in the discipline of electrical engineering without ever having to give much thought to other interdisciplinary fields .

have you ever sat in a doctor 's office for hours despite having an appointment at a specific time ? has a hotel turned down your reservation because it 's full ? or have you been bumped off a flight that you paid for ? these are all symptoms of overbooking , a practice where businesses and institutions sell or book more than their full capacity . while often infuriating for the customer , overbooking happens because it increases profits while also letting businesses optimize their resources . they know that not everyone will show up to their appointments , reservations , and flights , so they make more available than they actually have to offer . airlines are the classical example , partially because it happens so often . about 50,000 people get bumped off their flights each year . that figure comes at little surprise to the airlines themselves , which use statistics to determine exactly how many tickets to sell . it 's a delicate operation . sell too few , and they 're wasting seats . sell too many , and they pay penalties - money , free flights , hotel stays , and annoyed customers . so here 's a simplified version of how their calculations work . airlines have collected years worth of information about who does and does n't show up for certain flights . they know , for example , that on a particular route , the probability that each individual customer will show up on time is 90 % . for the sake of simplicity , we 'll assume that every customer is traveling individually rather than as families or groups . then , if there are 180 seats on the plane and they sell 180 tickets , the most likely result is that 162 passengers will board . but , of course , you could also end up with more passengers , or fewer . the probability for each value is given by what 's called a binomial distribution , which peaks at the most likely outcome . now let 's look at the revenue . the airline makes money from each ticket buyer and loses money for each person who gets bumped . let 's say a ticket costs $ 250 and is n't exchangeable for a later flight . and the cost of bumping a passenger is $ 800 . these numbers are just for the sake of example . actual amounts vary considerably . so here , if you do n't sell any extra tickets , you make $ 45,000 . if you sell 15 extras and at least 15 people are no shows , you make $ 48,750 . that 's the best case . in the worst case , everyone shows up . 15 unlucky passengers get bumped , and the revenue will only be $ 36,750 , even less than if you only sold 180 tickets in the first place . but what matters is n't just how good or bad a scenario is financially , but how likely it is to happen . so how likely is each scenario ? we can find out by using the binomial distribution . in this example , the probability of exactly 195 passengers boarding is almost 0 % . the probability of exactly 184 passengers boarding is 1.11 % , and so on . multiply these probabilities by the revenue for each case , add them all up , and subtract the sum from the earnings by 195 sold tickets , and you get the expected revenue for selling 195 tickets . by repeating this calculation for various numbers of extra tickets , the airline can find the one likely to yield the highest revenue . in this example , that 's 198 tickets , from which the airline will probably make $ 48,774 , almost 4,000 more than without overbooking . and that 's just for one flight . multiply that by a million flights per airline per year , and overbooking adds up fast . of course , the actual calculation is much more complicated . airlines apply many factors to create even more accurate models . but should they ? some argue that overbooking is unethical . you 're charging two people for the same resource . of course , if you 're 100 % sure someone wo n't show up , it 's fine to sell their seat . but what if you 're only 95 % sure ? 75 % ? is there a number that separates being unethical from being practical ?

by repeating this calculation for various numbers of extra tickets , the airline can find the one likely to yield the highest revenue . in this example , that 's 198 tickets , from which the airline will probably make $ 48,774 , almost 4,000 more than without overbooking . and that 's just for one flight .

overbooking strategies use … to make predictions .

some of the best opportunities to learn are the moments in which we are perplexed . those moments in which you begin to wonder and question . these moments have happened throughout history . and have led to some truly amazing discoveries . take this story , for example . there once was a fellow named archimedes . he was born in 287 b.c . in the city of syracuse in sicily . he was a greek mathematician , physicist , engineer , inventor , and astronomer . one day , archimedes was summoned by the king of sicily to investigate if he had been cheated by a goldsmith . the king said he had given a goldsmith the exact amount of gold needed to make a crown . however , when the crown was ready , the king suspected that the goldsmith cheated and slipped some silver into the crown , keeping some of the gold for himself . the king asked archimedes to solve the problem . but there was a catch : he could n't do any damage to the crown . one day , while taking his bath , archimedes noticed that the water level in the bathtub rose and overflowed as he immersed himself into the tub . he suddenly realized that how much water was displaced depended on how much of his body was immersed . this discovery excited him so much that he jumped out of the tub and ran through the streets naked , shouting `` eureka ! '' which comes from the ancient greek meaning `` i found it . '' what did he find ? well , he found a way to solve the king 's problem . you see , archimedes needed to check the crown 's density to see if it was the same as the density of pure gold . density is a measure of an object 's mass divided by its volume . pure gold is very dense , while silver is less dense . so if there was silver in the crown , it would be less dense than if it were made of pure gold . but no matter what it was made of , the crown would be the same shape , which means the same volume . so if archimedes could measure the mass of the crown first , and then measure its volume , he could find out how dense it was . but it is not easy to measure a crown 's volume - it has an irregular shape , that 's different from a simple box or ball . you ca n't measure its size and multiply like you might for other shapes . the solution , archimedes realized , was to give the crown a bath . by placing it in water and seeing how much water was displaced , he could measure the volume , and he 'd calculate the density of the crown . if the crown was less dense than pure gold , then the goldsmith most definitely cheated the king . when archimedes went back to the king and did his test , the story says , he found that the goldsmith had indeed cheated the king , and slipped some silver in . these days , using the way an object displaces water to measure volume is called archimedes ' principle . the next time you take a bath , you can see archimedes ' principle in action , and maybe you 'll have a genius idea of your own .

take this story , for example . there once was a fellow named archimedes . he was born in 287 b.c . in the city of syracuse in sicily .

when and where was archimedes born ?

translator : andrea mcdonough reviewer : bedirhan cinar have you ever wondered how things are built within our bodies ? why our bodies can regrow and repair themselves , and how we can pass on genes from one generation to the next ? yet , none of our man-made objects have these traits ; they 're simply thrown away when they break and they definitely ca n't reproduce . the answer lies in something called self assembly . self assembly is a system where unordered parts come together in an organized structure , completely on their own . this means that a pile of parts on your desk should , in theory , be able to move around on their own , find one another , and build something useful . this seems impossible , like transformers or the sandman , but it 's exactly how our bodies are built , how our immune system works , and why we can reproduce . self assembly is the factory and copy machines within our bodies that make proteins fold and dna replicate . it 's a process that not only happens in the biological and chemical world , but is a phenomenon that can be seen from magnets to snowflakes , robotics , social networks , the formations of cities and galaxies , to name just a few . in biology and chemistry , self assembly is everywhere , from atomic interactions , cellular replication to dna , rna , and protein folding . proteins are like bicycle chains with sequences of amino acid links . they self assemble into 3-d structures because of the interaction between the amino acids along the chain , as well as the relationship between the chain and the environment . these forces make the flexible chain fold into a 3-d shape that governs the function in the protein . viruses , on the other hand , are like soccer balls . they 're made up of a series of sub-units with specific shapes . those shapes have attraction to one another , so they fit together in precise ways . image you want to build a perfect sphere . it turns out that making a precise sphere through traditional means is actually quite difficult . alternatively , you could try to self assemble the sphere . one way would be to inflate the sphere like a bubble or a balloon . another option would be to create many identical pieces that would come together to make a perfect sphere . you could try to put the pieces together one-by-one , but it might take a long time and you would still have human errors . instead you could design a connection between the components like magnets and dump them into a container . when you shook the container , all the parts would find one another and build the sphere for you . self assembly is being used as a new design , science , and engineering tool for making the next generation of technologies easier to build , more adaptive , and less reliant on fossil fuels . scientists are now making molecular microchips for computers where small , molecular elements are given the right conditions to form themselves into organized pathways . similarly , we can now use self assembly as a way to make 3-d structures with dna , like capsules that could deliver drugs inside the body , releasing them only if certain conditions are met . soon , self assembly will be used for larger applications , where materials can repair themselves , water pipes can reconfigure on demand , buildings can adapt on their own to environment or dynamic loading , and space structures can self assemble without humans . imagine if our factories were more like organisms or brains and our construction sites were like gardens that grow and adapt independently . the possibilities are endless and it 's now up to us to design a better world through self assembly .

yet , none of our man-made objects have these traits ; they 're simply thrown away when they break and they definitely ca n't reproduce . the answer lies in something called self assembly . self assembly is a system where unordered parts come together in an organized structure , completely on their own .

where do we see self-assembly in biology ?

professor fukanō , the famous eccentric scientist and adventurer , has embarked on a new challenge : flying around the world nonstop in a plane of his own design . able to travel consistently at the incredible speed of one degree longitude around the equator per minute , the plane would take six hours to circle the world . there 's just one problem : the plane can only hold 180 kiloliters of fuel , only enough for exactly half the journey . let 's be honest . the professor probably could have designed the plane to hold more fuel , but where 's the fun in that ? instead , he 's devised a slightly more elaborate solution : building three identical planes for the mission . in addition to their speed , the professor 's equipped them with a few other incredible features . each of the planes can turn on a dime and instantly transfer any amount of its fuel to any of the others in midair without slowing down , provided they 're next to each other . the professor will pilot the first plane , while his two assistants fugōri and orokana will pilot each of the others . however , only one airport , located on the equator , has granted permission for the experiment , making it the starting point , the finish line , and the only spot where the planes can land , takeoff , or refuel on the ground . how should the three planes coordinate so the professor can fly continuously for the whole trip and achieve his dream without anyone running out of fuel and crashing ? pause here if you want to figure it out for yourself . answer in : 3 answer in : 2 answer in : 1 according to the professor 's calculations , they should be able to pull it off by a hair . the key is to maximize the support each assistant provides , not wasting a single kiloliter of fuel . it also helps us to think symmetrically so they can make shorter trips in either direction while setting the professor up for a long unsupported stretch in the middle . here 's his solution . all three planes take off at noon flying west , each fully loaded with 180 kiloliters . after 45 minutes , or one-eighth of the way around , each plane has 135 kiloliters left . orokana gives 45 to the professor and 45 to fugōri , fully refueling them both . with her remaining 45 , orokana returns to the airport and heads to the lounge for a well-deserved break . 45 minutes later , with one-quarter of the trip complete , the professor and fugōri are both at 135 kiloliters again . fugōri transfers 45 into the professor 's tank , leaving himself with the 90 he needs to return . professor fukanō stretches and puts on his favorite album . he 'll be alone for a while . in the meantime , orokana has been anxiously awaiting fugōri 's return , her plane fully refueled and ready to go . as soon as his plane touches the ground , she takes off , this time flying east . at this point , exactly 180 minutes have passed and the professor is at the halfway point of his journey with 90 kiloliters of fuel left . for the next 90 minutes , the professor and orokana 's planes fly towards each other , meeting at the three-quarter mark . just as the professor 's fuel is about the run out , he sees orokana 's plane . she gives him 45 kiloliters of her remaining 90 , leaving them with 45 each . but that 's just half of what they need to make it to the airport . fortunately , this is exactly when fugōri , having refueled , takes off . 45 minutes later , just as the other two planes are about to run empty , he meets them at the 315 degree point and transfers 45 kiloliters of fuel to each , leaving 45 for himself . all three planes land at the airport just as their fuel gauges reach zero . as the reporters and photographers cheer , the professor promises his planes will soon be available for commercial flights , just as soon as they figure out how to keep their inflight meals from spilling everywhere .

professor fukanō , the famous eccentric scientist and adventurer , has embarked on a new challenge : flying around the world nonstop in a plane of his own design . able to travel consistently at the incredible speed of one degree longitude around the equator per minute , the plane would take six hours to circle the world . there 's just one problem : the plane can only hold 180 kiloliters of fuel , only enough for exactly half the journey .

the equator :

translator : tom carter reviewer : bedirhan cinar as you can imagine , 400 years ago , navigating the open ocean was difficult . the winds and currents pushed and pulled ships off course , and so sailors based their directions on the port they left , attempting to maintain an accurate record of the ship 's direction and the distance sailed . this process was known as dead reckoning , because being just half a degree off could result in sailing right past the island that lay several miles just over the horizon . this was an easy mistake to make . thankfully , three inventions made modern navigation possible : sextants , clocks and the mathematics necessary to perform the required calculations quickly and easily . all are important . without the right tools , many sailors would be reluctant to sail too far from the sight of land . john bird , an instrument maker in london , made the first device that could measure the angle between the sun and the horizon during the day , called a sextant . knowing this angle was important , because it could be compared to the angle back in england at the exact same time . comparing these two angles was necessary to determine the longitude of the ship . clocks came next . in 1761 , john harrison , an english clockmaker and carpenter , built a clock that could keep accurate time at sea . the timepiece that could maintain accurate time while on a pitching , yawing deck in harsh conditions was necessary in order to know the time back in england . there was one catch though : since such a timepiece was handmade , it was very expensive . so an alternate method using lunar measurements and intense calculations was often used to cut costs . the calculations to determine a ship 's location for each measurement could take hours . but sextants and clocks were n't useful unless sailors could use these tools to determine their position . fortunately , in the 1600s , an amateur mathematician had invented the missing piece . john napier toiled for more than 20 years in his castle in scotland to develop logarithms , a calculation device . napier 's ideas on logarithms involved the form of one over e and the constant 10 to the seventh power . algebra in the early 1600s was not fully developed , and napier 's logarithm of one did not equal zero . this made the calculations much less convenient than logarithms with a base of 10 . henry briggs , a famous mathematician at gresham college in london , read napier 's work in 1614 , and the following year made the long journey to edinburgh to meet napier . briggs showed up unannounced at napier 's castle door and suggested that john switch the base and form of his logarithms into something much simpler . they both agreed that a base of 10 with the log of one equal to zero would greatly simplify everyday calculations . today we remember these as briggs common logarithms . until the development of electric calculating machines in the 20th century , any calculations involving multiplication , division , powers , and extraction of roots with large and small numbers were done using logarithms . the history of logarithms is n't just a lesson in math . there were many players responsible for successful navigation . instrument makers , astronomers , mathematicians , and of course sailors . creativity is n't only about going deep into one 's field of work , it 's about cross-pollination between disciplines too .

translator : tom carter reviewer : bedirhan cinar as you can imagine , 400 years ago , navigating the open ocean was difficult . the winds and currents pushed and pulled ships off course , and so sailors based their directions on the port they left , attempting to maintain an accurate record of the ship 's direction and the distance sailed . this process was known as dead reckoning , because being just half a degree off could result in sailing right past the island that lay several miles just over the horizon .

approximately 400 years ago , ships attempted to maintain an accurate record of direction , time , and distance . sailors based their directions on the port they left . this process is called :

imagine as a thought experiment that you live in a small village and depend on the local fish pond for food . you share the pond with three other villagers . the pond starts off with a dozen fish , and the fish reproduce . for every two fish , there will be one baby added each night . so , in order to maximize your supply of food , how many fish should you catch each day ? take a moment to think about it . assume baby fish grow to full size immediately and that the pond begins at full capacity , and ignore factors like the sex of the fish you catch . the answer ? one , and it 's not just you . the best way to maximize every villager 's food supply is for each fisherman to take just one fish each day . here 's how the math works . if each villager takes one fish , there will be eight fish left over night . each pair of fish produces one baby , and the next day , the pond will be fully restocked with twelve fish . if anyone takes more than one , the number of reproductive pairs drops , and the population wo n't be able to bounce back . eventually , the fish in the lake will be gone , leaving all four villagers to starve . this fish pond is just one example of a classic problem called the tragedy of the commons . the phenomenon was first described in a pamphlet by economist william forster lloyd in 1833 in a discussion of the overgrazing of cattle on village common areas . more than 100 years later , ecologist garrett hardin revived the concept to describe what happens when many individuals all share a limited resource , like grazing land , fishing areas , living space , even clean air . hardin argued that these situations pit short-term self-interest against the common good , and they end badly for everyone , resulting in overgrazing , overfishing , overpopulation , pollution , and other social and environmental problems . the key feature of a tragedy of the commons is that it provides an opportunity for an individual to benefit him or herself while spreading out any negative effects across the larger population . to see what that means , let 's revisit our fish pond . each individual fisherman is motivated to take as many fish as he can for himself . meanwhile , any decline in fish reproduction is shared by the entire village . anxious to avoid losing out to his neighbors , a fisherman will conclude that it 's in his best interest to take an extra fish , or two , or three . unfortunately , this is the same conclusion reached by the other fisherman , and that 's the tragedy . optimizing for the self in the short term is n't optimal for anyone in the long term . that 's a simplified example , but the tragedy of the commons plays out in the more complex systems of real life , too . the overuse of antibiotics has led to short-term gains in livestock production and in treating common illnesses , but it 's also resulted in the evolution of antibiotic-resistant bacteria , which threaten the entire population . a coal-fired power plant produces cheap electricity for its customers and profits for its owners . these local benefits are helpful in the short term , but pollution from mining and burning coal is spread across the entire atmosphere and sticks around for thousands of years . there are other examples , too . littering , water shortages , deforestation , traffic jams , even the purchase of bottled water . but human civilization has proven it 's capable of doing something remarkable . we form social contracts , we make communal agreements , we elect governments , and we pass laws . all this to save our collective selves from our own individual impulses . it is n't easy , and we certainly do n't get it right nearly all of the time . but humans at our best have shown that we can solve these problems and we can continue to do so if we remember hardin 's lesson . when the tragedy of the commons applies , what 's good for all of us is good for each of us .

eventually , the fish in the lake will be gone , leaving all four villagers to starve . this fish pond is just one example of a classic problem called the tragedy of the commons . the phenomenon was first described in a pamphlet by economist william forster lloyd in 1833 in a discussion of the overgrazing of cattle on village common areas .

explain how overfishing is an example of the tragedy of the commons .

in 1956 , during a diplomatic reception in moscow , soviet leader nikita khrushchev told western bloc ambassadors , `` my vas pokhoronim ! '' his interpreter rendered that into english as , `` we will bury you ! '' this statement sent shockwaves through the western world , heightening the tension between the soviet union and the us who were in the thick of the cold war . some believe this incident alone set east/west relations back a decade . as it turns out , khrushchev 's remark was translated a bit too literally . given the context , his words should have been rendered as , `` we will live to see you buried , '' meaning that communism would outlast capitalism , a less threatening comment . though the intended meaning was eventually clarified , the initial impact of khrushchev 's apparent words put the world on a path that could have led to nuclear armageddon . so now , given the complexities of language and cultural exchange , how does this sort of thing not happen all the time ? much of the answer lies with the skill and training of interpreters to overcome language barriers . for most of history , interpretation was mainly done consecutively , with speakers and interpreters making pauses to allow each other to speak . but after the advent of radio technology , a new simultaneous interpretations system was developed in the wake of world war ii . in the simultaneous mode interpreters instantaneously translate a speaker 's words into a microphone while he speaks . without pauses , those in the audience can choose the language in which they want to follow . on the surface , it all looks seamless , but behind the scenes , human interpreters work incessantly to ensure every idea gets across as intended . and that is no easy task . it takes about two years of training for already fluent bilingual professionals to expand their vocabulary and master the skills necessary to become a conference interpreter . to get used to the unnatural task of speaking while they listen , students shadow speakers and repeat their every word exactly as heard in the same language . in time , they begin to paraphrase what is said , making stylistic adjustments as they go . at some point , a second language is introduced . practicing in this way creates new neural pathways in the interpreter 's brain , and the constant effort of reformulation gradually becomes second nature . over time and through much hard work , the interpreter masters a vast array of tricks to keep up with speed , deal with challenging terminology , and handle a multitude of foreign accents . they may resort to acronyms to shorten long names , choose generic terms over specific , or refer to slides and other visual aides . they can even leave a term in the original language , while they search for the most accurate equivalent . interpreters are also skilled at keeping aplomb in the face of chaos . remember , they have no control over who is going to say what , or how articulate the speaker will sound . a curveball can be thrown at any time . also , they often perform to thousands of people and in very intimidating settings , like the un general assembly . to keep their emotions in check , they carefully prepare for an assignment , building glossaries in advance , reading voraciously about the subject matter , and reviewing previous talks on the topic . finally , interpreters work in pairs . while one colleague is busy translating incoming speeches in real time , the other gives support by locating documents , looking up words , and tracking down pertinent information . because simultaneous interpretation requires intense concentration , every 30 minutes , the pair switches roles . success is heavily dependent on skillful collaboration . language is complex , and when abstract or nuanced concepts get lost in translation , the consequences may be catastrophic . as margaret atwood famously noted , `` war is what happens when language fails . '' conference interpreters of all people are aware of that and work diligently behind the scenes to make sure it never does .

to keep their emotions in check , they carefully prepare for an assignment , building glossaries in advance , reading voraciously about the subject matter , and reviewing previous talks on the topic . finally , interpreters work in pairs . while one colleague is busy translating incoming speeches in real time , the other gives support by locating documents , looking up words , and tracking down pertinent information .

for conference interpreters , effective stress management skills are as important as linguistic competence . list three things interpreters do to keep their emotions in check .

translator : andrea mcdonough reviewer : jessica ruby in the 11th and 12th centuries , most english commoners were illiterate . since they had no way to learn the bible , the clergy came up with an inventive solution : they 'd create plays out of certain bible stories so even people who could n't read could learn them . these were called mystery plays because they revealed the mystery of god 's word . at about the same time , the clergy also developed plays about the saints of the church , called miracle plays . in the beginning , the clergy members acted out bible stories on the steps outside the cathedral . the audience reacted so well that soon they needed to move out to the street around the town square . by building moving carts to put on each play and by lining up one after the other , they could put on cycles of stories , which would take the viewer from genesis to revelation . these movable carts , called pageants , looked like huge boxes on wheels . each was two stories tall . the bottom story was curtained off and was used for costumes , props , and dressing . the top platform was the stage for the performance . spectators assembled in various corners of the town , and the pageant would move around in the cycle until the villagers had seen the entire series . soon , the plays required more actors than the clergy could supply . so , by the 13th century , different guilds were asked to be responsible for acting out different parts of the cycle . the assignments were meant to reflect the guilds ' professions . for example , the carpenter 's guild might put on the story of noah 's ark , and the baker 's guild might put on the last supper . can you imagine what might happen to the story if the butcher 's guild put on the crucifixion of christ ? yes , without the clergy , the plays soon started changing from their true bible stories . by the end of the 14th century , a new form of drama , called the morality play , had evolved . faith , truth , charity , and good deeds all became characters on the stage . and , at the same time , the opposite virtues of falsehood , covetousness , worldly flesh , and the devil became the antagonists . the morality plays were allegorical stories in which these characters battled for the control of the soul . audiences loved the immoral characters , and spectators were encouraged to interact with the actors . throwing rotten food and even getting into scuffles with other spectators became very common . the character of the devil often would roam through the crowds and pull unsuspecting watchers into a hell that was depicted as a dragon 's mouth . the virtuous biblical stories had morphed into crude and sometimes comic stories . the clergy intended to teach against immorality . how ironic , then , that the morality plays actually encouraged vices as more popular than virtues . by the mid-15th century , the church started to outlaw these performances . town charters required that any theater must be built outside the city wall . one of the first theaters was built like a larger version of a pageant , with tiers of gallery seating encircling a grassy area in front of the stage . sound familiar ? a young william shakespeare developed his craft here at the theater that was eventually renamed the globe . the medieval morality play had led to renaissance playwrights who were inspired by the inner struggles and the conscience of man . and that , in essence , is how drama emerged as a literary art form .

the medieval morality play had led to renaissance playwrights who were inspired by the inner struggles and the conscience of man . and that , in essence , is how drama emerged as a literary art form .

what group began drama in england ?

there are many stories that can be told about world war ii , from the tragic to the inspring . but perhaps one of the most heartrending experiences was that of the akune family , divided by the war against each other and against their own identities . ichiro akune and his wife yukiye immigrated to america from japan in 1918 in search of opportunity , opening a small grocery store in central california and raising nine children . but when mrs. akune died in 1933 , the children were sent to live with relatives in japan , their father following soon after . though the move was a difficult adjustment after having been born and raised in america , the oldest son , harry , formed a close bond with his grand uncle , who taught him the japanese language , culture and values . nevertheless , as soon as harry and his brother ken were old enough to work , they returned to the country they considered home , settling near los angeles . but then , december 7 , 1941 , the attack on pearl harbor . now at war with japan , the united states government did not trust the loyalty of those citizens who had family or ancestral ties to the enemy country . in 1942 , about 120,000 japanese americans living on the west coast were stripped of their civil rights and forcibly relocated to internment camps , even though most of them , like harry and ken , were nisei , american or dual citizens who had been born in the us to japanese immigrant parents . the brothers not only had very limited contact with their family in japan , but found themselves confined to a camp in a remote part of colorado . but their story took another twist when recruiters from the us army 's military intelligence service arrived at the camp looking for japanese-speaking volunteers . despite their treatment by the government , harry and ken jumped at the chance to leave the camp and prove their loyalty as american citizens . having been schooled in japan , they soon began their service , translating captured documents , interrogating japanese soldiers , and producing japanese language propaganda aimed at persuading enemy forces to surrender . the brothers ' work was invaluable to the war effort , providing vital strategic information about the size and location of japanese forces . but they still faced discrimination and mistrust from their fellow soldiers . harry recalled an instance where his combat gear was mysteriously misplaced just prior to parachuting into enemy territory , with the white officer reluctant to give him a weapon . nevertheless , both brothers continued to serve loyally through the end of the war . but harry and ken were not the only akune brothers fighting in the pacific . unbeknownst to them , two younger brothers , the third and fourth of the five akune boys , were serving dutifully in the imperial japanese navy , saburo in the naval airforce , and 15-year-old shiro as an orientation trainer for new recruits . when the war ended , harry and ken served in the allied occupational forces and were seen as traitors by the locals . when all the akune brothers gathered at a family reunion in kagoshima for the first time in a decade , it was revealed that the two pairs had fought on opposing sides . tempers flared and a fight almost broke out until their father stepped in . the brothers managed to make peace and saburo and shiro joined harry and ken in california , and later fought for the us army in korea . it took until 1988 for the us government to acknowledge the injustice of its internment camps and approve reparations payments to survivors . for harry , though , his greatest regret was not having the courage to thank his japanese grand uncle who had taught him so much . the story of the akune brothers is many things : a family divided by circumstance , the unjust treatment of japanese americans , and the personal struggle of reconciling two national identities . but it also reveals a larger story about american history : the oppression faced by immigrant groups and their perseverance in overcoming it .

it took until 1988 for the us government to acknowledge the injustice of its internment camps and approve reparations payments to survivors . for harry , though , his greatest regret was not having the courage to thank his japanese grand uncle who had taught him so much . the story of the akune brothers is many things : a family divided by circumstance , the unjust treatment of japanese americans , and the personal struggle of reconciling two national identities .

harry akune tearfully regretted not having the courage to thank his grand-uncle after the war . what did the grand-uncle do for harry ?

ani stands before a large golden scale where the jackal-headed god anubis is weighing his heart against a pure ostrich feather . ani was a real person , a scribe from the egyptian city of thebes who lived in the 13th century bce . and depicted here is a scene from his book of the dead , a 78-foot papyrus scroll designed to help him attain immortality . such funerary texts were originally written only for pharaohs , but with time , the egyptians came to believe regular people could also reach the afterlife if they succeeded in the passage . ani 's epic journey begins with his death . his body is mummified by a team of priests who remove every organ except the heart , the seat of emotion , memory , and intelligence . it 's then stuffed with a salt called natron and wrapped in resin-soaked linen . in addition , the wrappings are woven with charms for protection and topped with a heart scarab amulet that will prove important later on . the goal of the two-month process is to preserve ani 's body as an ideal form with which his spirit can eventually reunite . but first , that spirit must pass through the duat , or underworld . this is a realm of vast caverns , lakes of fire , and magical gates , all guarded by fearsome beasts - snakes , crocodiles , and half-human monstrosities with names like `` he who dances in blood . '' to make things worse , apep , the serpent god of destruction , lurks in the shadows waiting to swallow ani 's soul . fortunately , ani is prepared with the magic contained within his book of the dead . like other egyptians who could afford it , ani customized his scroll to include the particular spells , prayers , and codes he thought his spirit might need . equipped with this arsenal , our hero traverses the obstacles , repels the monsters ' acts , and stealthily avoids apep to reach the hall of ma'at , goddess of truth and justice . here , ani faces his final challenge . he is judged by 42 assessor gods who must be convinced that he has lived a righteous life . ani approaches each one , addressing them by name , and declaring a sin he has not committed . among these negative confessions , or declarations of innocence , he proclaims that he has not made anyone cry , is not an eavesdropper , and has not polluted the water . but did ani really live such a perfect life ? not quite , but that 's where the heart scarab amulet comes in . it 's inscribed with the words , `` do not stand as a witness against me , '' precisely so ani 's heart does n't betray him by recalling the time he listened to his neighbors fight or washed his feet in the nile . now , it 's ani 's moment of truth , the weighing of the heart . if his heart is heavier than the feather , weighed down by ani 's wrongdoings , it 'll be devoured by the monstrous ammit , part crocodile , part leopard , part hippopotamus , and ani will cease to exist forever . but ani is in luck . his heart is judged pure . ra , the sun god , takes him to osiris , god of the underworld , who gives him final approval to enter the afterlife . in the endless and lush field of reeds , ani meets his deceased parents . here , there is no sadness , pain , or anger , but there is work to be done . like everyone else , ani must cultivate a plot of land , which he does with the help of a shabti doll that had been placed in his tomb . today , the papyrus of ani resides in the british museum , where it has been since 1888 . only ani , if anyone , knows what really happened after his death . but thanks to his book of the dead , we can imagine him happily tending his crops for all eternity .

ani was a real person , a scribe from the egyptian city of thebes who lived in the 13th century bce . and depicted here is a scene from his book of the dead , a 78-foot papyrus scroll designed to help him attain immortality . such funerary texts were originally written only for pharaohs , but with time , the egyptians came to believe regular people could also reach the afterlife if they succeeded in the passage .

how does a `` book of the dead '' help someone reach the afterlife ?

he was one of the most influential figures of the 20th century , forever changing the course of one of the world 's largest countries . but was he a hero who toppled an oppressive tyranny or a villain who replaced it with another ? it 's time to put lenin on the stand in history vs. lenin . `` order , order , hmm . now , was n't it your fault that the band broke up ? '' `` your honor , this is vladimir ilyich ulyanov , aka lenin , the rabblerouser who helped overthrow the russian tsar nicholas ii in 1917 and founded the soviet union , one of the worst dictatorships of the 20th century . '' `` ohh . '' `` the tsar was a bloody tyrant under whom the masses toiled in slavery . '' `` this is rubbish . serfdom had already been abolished in 1861 . '' `` and replaced by something worse . the factory bosses treated the people far worse than their former feudal landlords . and unlike the landlords , they were always there . russian workers toiled for eleven hours a day and were the lowest paid in all of europe . '' `` but tsar nicholas made laws to protect the workers . '' `` he reluctantly did the bare minimum to avert revolution , and even there , he failed . remember what happened in 1905 after his troops fired on peaceful petitioners ? '' `` yes , and the tsar ended the rebellion by introducing a constitution and an elected parliament , the duma . '' `` while retaining absolute power and dissolving them whenever he wanted . '' `` perhaps there would 've been more reforms in due time if radicals , like lenin , were n't always stirring up trouble . '' `` your honor , lenin had seen his older brother aleksandr executed by the previous tsar for revolutionary activity , and even after the reforms , nicholas continued the same mass repression and executions , as well as the unpopular involvement in world war i , that cost russia so many lives and resources . '' `` hm , this tsar does n't sound like such a capital fellow . '' `` your honor , maybe nicholas ii did doom himself with bad decisions , but lenin deserves no credit for this . when the february 1917 uprisings finally forced the tsar to abdicate , lenin was still exiled in switzerland . '' `` hm , so who came to power ? '' `` the duma formed a provisional government , led by alexander kerensky , an incompetent bourgeois failure . he even launched another failed offensive in the war , where russia had already lost so much , instead of ending it like the people wanted . '' `` it was a constitutional social democratic government , the most progressive of its time . and it could have succeeded eventually if lenin had n't returned in april , sent by the germans to undermine the russian war effort and instigate riots . '' `` such slander ! the july days were a spontaneous and justified reaction against the government 's failures . and kerensky showed his true colors when he blamed lenin and arrested and outlawed his bolshevik party , forcing him to flee into exile again . some democracy ! it 's a good thing the government collapsed under their own incompetence and greed when they tried to stage a military coup then had to ask the bolsheviks for help when it backfired . after that , all lenin had to do was return in october and take charge . the government was peacefully overthrown overnight . '' `` but what the bolsheviks did after gaining power was n't very peaceful . how many people did they execute without trial ? and was it really necessary to murder the tsar 's entire family , even the children ? '' `` russia was being attacked by foreign imperialists , trying to restore the tsar . any royal heir that was rescued would be recognized as ruler by foreign governments . it would 've been the end of everything the people had fought so hard to achieve . besides , lenin may not have given the order . '' `` but it was not only imperialists that the bolsheviks killed . what about the purges and executions of other socialist and anarchist parties , their old allies ? what about the tambov rebellion , where peasants , resisting grain confiscation , were killed with poison gas ? or sending the army to crush the workers in kronstadt , who were demanding democratic self-management ? was this still fighting for the people ? '' `` yes ! the measures were difficult , but it was a difficult time . the new government needed to secure itself while being attacked from all sides , so that the socialist order could be established . '' `` and what good came of this socialist order ? even after the civil war was won , there were famines , repression and millions executed or sent to die in camps , while lenin 's successor stalin established a cult of personality and absolute power . '' `` that was n't the plan . lenin never cared for personal gains , even his enemies admitted that he fully believed in his cause , living modestly and working tirelessly from his student days until his too early death . he saw how power-hungry stalin was and tried to warn the party , but it was too late . '' `` and the decades of totalitarianism that followed after ? '' `` you could call it that , but it was lenin 's efforts that changed russia in a few decades from a backward and undeveloped monarchy full of illiterate peasants to a modern , industrial superpower , with one of the world 's best educated populations , unprecedented opportunities for women , and some of the most important scientific advancements of the century . life may not have been luxurious , but nearly everyone had a roof over their head and food on their plate , which few countries have achieved . '' `` but these advances could still have happened , even without lenin and the repressive regime he established . '' `` yes , and i could 've been a famous rock and roll singer . but how would i have sounded ? '' we can never be sure how things could 've unfolded if different people were in power or different decisions were made , but to avoid the mistakes of the past , we must always be willing to put historical figures on trial .

`` this is rubbish . serfdom had already been abolished in 1861 . '' `` and replaced by something worse .

when was serfdom abolished in russia ?

so we 're gon na go find some liquid nitrogen . we keep it in a really large dewar tank at the back of the shop , so we 're gon na go find that now . so this is a dewar that we used to keep the liquid nitrogen in within the building so it 's much more convenient for us and our students to get it . so what we 're gon na do now is we 're gon na fill up our small dewar to carry the nitrogen back to the lab . so you can probably hear it hissing and fizzing going into the bottle , and you can see all the nitrogen evaporating off . nitrogen exists as a gas . molecules of n2 . and it is interesting , because the bond between the two nitrogen atoms is the strongest bond that you can get between any two atoms in the periodic table . any two atoms that are the same . sometimes , if you have two atoms that are different , the bond can be slightly stronger , but the nn triple bond is one of the strongest that you can have . so what we 're doing is we 're filling up the dewar with liquid nitrogen so that we can take that back to our lab to do our next experiment . so many explosive compounds , things that you 've heard of , like tnt and other explosives , all contain nitrogen , which are isolated atoms , and when the explosion goes off the atoms come together and release energy . nitrogen is used widely in laboratories , partly because it 's not as reactive as oxygen , so if you do an experiment in an atmosphere of nitrogen you can avoid reactions that might occur with air . you can also use nitrogen when it 's liquified as a coolant . liquid nitrogen boils at -196 degrees centigrade , 77 degrees absolute , and you can just hold it in a normal thermous flask . so this is a dewar of liquid nitrogen , and you can see that the nitrogen is evaporating from the top , coming out and frosting , coming down over the sides because it 's a little bit more dense than the air . you can see it 's heavily laden with ice , and you can see that the ice crystals are starting to grow on the top of the dewar here . it 's very cold though , so we must n't touch it for too long . so if we just tip some out on the floor , we can see that the nitrogen itself will come out like a liquid , like pouring water , but the interesting thing is , water , when i pour it on the floor it forms a puddle . it does nothing useful . in fact , it sort of gets in the way . but liquid nitrogen , when i pour it on the floor , it forms these really quite nice bubbles , and they fly across to the side of the room and take all the dust with them . pretty cool . but if you put materials like rubber or flowers into liquid nitrogen , instead of being soft and bendy they become really rigid , and you can smash them with a hammer . so you can see all of the bubbles forming on this steel tray now , and they 're sweeping away all the dust quite nicely as they evaporate and move further afield . so neil 's got a piece of tubing , and it 's really quite flexible tubing , you can see it 's really floppy , and now he 's gon na put it into the cryogenic liquid nitrogen and freeze it , and see what happens to the rubber . but you see another interesting thing there is that the liquid nitrogen is siphoning out through the tube , and that 's a really good joke , because you can direct that siphon at someones trousers and freeze their legs . so i think that 's enough , neil , let 's see if it 's a bit harder . wow , that shattered . and there 's quite a nice little operation that i 've heard about , in which you can take the lens out of somebody 's eye when they 're suitably anaesthetized , and this lens can be frozen and then put on the lathe and you can change its shape so you get the right shape , and then you let it unfreeze , it goes soft again , and you pop it back into their eye . . . . . . . . . . . . . captions by www.subply.com

so we 're gon na go find some liquid nitrogen . we keep it in a really large dewar tank at the back of the shop , so we 're gon na go find that now .

what is the fog that can be seen around the dewar flask of liquid nitrogen ?

translator : jenny zurawell i am awele . daughter of alice , granddaughter of ruth , great-granddaughter of big momma alice and madir corine , great-great-granddaughter of anna and zitii benyen . it is my hope to find my best possible self in the service of others . now , my daddy , he used to tell me stories . my daddy , he would say , `` i want you to know who you are and where you come from . that will guide you as you discover who you must be . now , you listen to this story , you hear me , baby girl ? it 's not going to be in a book . your teacher is not going to tell it , but you need to understand who you are . '' that became a guiding principle in the stories that i wanted to tell . stories about legacy of who we are . i used to hear all the time that children are the future , but what does that cliché really mean and how are we preparing them ? so i looked for narratives about young people and the legacy that they bring as agents of change . the power that you have right now . today , march 2 , 1955 -- the story that i want to share with you comes from 1955 , march 2 . it 's about a courageous 16-year-old girl , claudette colvin . and it comes full circle today because a week ago today , in san francisco , my middle school students , they performed a program that i had written , `` agents of change , '' starting with the reenactment of plessy v. ferguson from 1892 to 1896 , moving to brown v. board and a student-led strike by barbara rose johns , jumping to claudette colvin and the montgomery bus boycott and ending in 1960 with the sit-in movement , the non-violent movement led by students . so i 'm going to share the story , and i would like to also share the work i do with it , as a case study . i paid my dime at the front of the bus , and then i ran to the back door with the rest of the colored kids so the driver would n't take off before we got on . also , well , whites do n't want us walking down the aisle next to them . when i got back on the bus , colored section was full , so , i sat in the middle section . i took the last row seat on the left , it was right by the window , was n't thinking about anything in particular . `` hey . '' i did n't know the girl next to me either , this older girl . so i just looked out the window . driver went more stops , more people were getting on , colored and white . pretty soon , no more seats were available . `` give me those seats , '' the driver called out . colored folks just started getting up . white folks started taking their seats , but i stayed seated . girl next to me and the other two across -- they stayed seated . i knew it was n't the restricted area . `` make light on your feet ! '' girl next to me got up immediately . she stood in the aisle , then the other two girls . but i told myself , this is n't the restricted area . the driver , he looked up , looked in the window , that mirror . he pulled over . a pregnant lady , mrs. hamilton , got on the bus . she ran to the back and got on , not knowing he was trying to have me relinquish my seat . and she sat right next to me . `` the two of you need to get up so i can drive on . '' `` sir , i paid my dime , i paid my fare . it 's my right , you know , my constitutional -- '' `` constitutional ? ha-ha , let me get the police . '' well he got off and he flagged down two motormen , and they came . and those motormen , they came onto the bus . looked at mrs. hamilton . `` now the two of you need to get up so the driver can drive on . '' `` sir , i paid my dime . i 'm pregnant . if i were to move right now , i 'd be very sick , sir . '' `` sir , i paid my dime too , you know , and it 's my right , my constitutional right . i 'm a citizen of the united states . you just read the 13th and 14th amendment , it 'll tell you so . i know the law . my teacher , she taught it at school . '' you see , my teacher , she taught the constitution , the bill of rights , the declaration of independence , patrick henry 's speech -- i even memorized it . my teacher , she would prick our minds , trying to see what we thinking about . she would say , `` who are you ? hmm ? who are you , sitting right here right now ? the person that people think they see from your outside ? who are you on the inside ? how you think ? how you feel ? what you believe ? would you be willing to stand up for what you believe in even if someone wants to hold you back because you 're different ? do you love your beautiful brown skin , children ? hmm ? are you american ? what does it mean to be an american ? huh ? homework tonight , write me an essay : `` what does it mean to be an american ? '' you need to know who you are , children ! '' my teacher , she would teach us history and current events . she said that 's how we can understand everything that 's going on and we can do something about it . `` sir , all i know is i hate jim crow . i also know if i ai n't got nothing worth living for , i ai n't got nothing worth dying for . so give me liberty or give me death ! ouch ! i do n't care ! take me to jail . '' they dragged her off the bus . next thing , claudette colvin was in a car seat , backseat of the police car , handcuffed through the windows . the following year , may 11 , 1956 , claudette colvin was the star witness in the federal court case browder v. gayle . her , an 18-year-old teenager and two others , women , mrs. browder . their case , browder v. gayle , went up to the supreme court . on the heels of brown v. board of education , the 14th amendment and her powerful testimony that day , the rest is history . now , why is it we do n't know this story ? the montgomery bus boycott -- we hear rosa parks , martin luther king , they will forever be lifted up . but the role women played in that movement , the role of claudette , as an up-stander , it teaches us important lessons that challenge us today . what does it mean to be a participant ? a responsible citizen in a democracy ? and lessons of courage and of faith ? so i find freedom movement history that includes young people so that they can explore these big ideas of identity , your chosen identity , and the imposed identity . what does membership in society mean ? who has it ? how do we make amends ? race and violence in america , as well as participatory citizenship . so these stories allow me to have conversations , to speak the unspeakable , that many are afraid to have . once in eugene , oregon , a young , blond-haired , blue-eyed boy , middle schooler , at the end of a performance in the dialogue said , `` but ms. awele , racism 's over , right ? '' and not wanting to answer for him , i said , `` turn to the person sitting next to you . see if you can come up with evidence . '' and i gave them four minutes to talk . soon , they began to tell stories , evidence of racism in their community . a girl wrote to me , a high school student in san francisco : `` i was going to skip school but then i heard we had an assembly , so i came . and after listening to the students talk and seeing your performance , i thought i should organize my friends and we should go down to a board meeting and tell them that want to have advanced classes for a through g requirements . '' so , i tell you this story today in honor of the legacy of young people that have come before , so that they will have guideposts and signs to be the change that they want to see in this world , as claudette colvin was . because she struck down the constitutionality of segregated seats in montgomery , alabama . thank you . ( applause ) thank you . ( applause )

so i find freedom movement history that includes young people so that they can explore these big ideas of identity , your chosen identity , and the imposed identity . what does membership in society mean ? who has it ?

makeba wants people to wrestle with tough but essential questions like “ what does membership in society mean ? who has it ? ” and “ what does it mean to be a responsible citizen in a democracy ? ” choose one of these questions and develop a brief response .

let 's say you 're on a game show . you 've already earned $ 1000 in the first round when you land on the bonus space . now , you have a choice . you can either take a $ 500 bonus guaranteed or you can flip a coin . if it 's heads , you win $ 1000 bonus . if it 's tails , you get no bonus at all . in the second round , you 've earned $ 2000 when you land on the penalty space . now you have another choice . you can either take a $ 500 loss , or try your luck at the coin flip . if it 's heads , you lose nothing , but if it 's tails , you lose $ 1000 instead . if you 're like most people , you probably chose to take the guaranteed bonus in the first round and flip the coin in the second round . but if you think about it , this makes no sense . the odds and outcomes in both rounds are exactly the same . so why does the second round seem much scarier ? the answer lies in a phenomenon known as loss aversion . under rational economic theory , our decisions should follow a simple mathematical equation that weighs the level of risk against the amount at stake . but studies have found that for many people , the negative psychological impact we feel from losing something is about twice as strong as the positive impact of gaining the same thing . loss aversion is one cognitive bias that arises from heuristics , problem-solving approaches based on previous experience and intuition rather than careful analysis . and these mental shortcuts can lead to irrational decisions , not like falling in love or bungee jumping off a cliff , but logical fallacies that can easily be proven wrong . situations involving probability are notoriously bad for applying heuristics . for instance , say you were to roll a die with four green faces and two red faces twenty times . you can choose one of the following sequences of rolls , and if it shows up , you 'll win $ 25 . which would you pick ? in one study , 65 % of the participants who were all college students chose sequence b even though a is shorter and contained within b , in other words , more likely . this is what 's called a conjunction fallacy . here , we expect to see more green rolls , so our brains can trick us into picking the less likely option . heuristics are also terrible at dealing with numbers in general . in one example , students were split into two groups . the first group was asked whether mahatma gandhi died before or after age 9 , while the second was asked whether he died before or after age 140 . both numbers were obviously way off , but when the students were then asked to guess the actual age at which he died , the first group 's answers averaged to 50 while the second group 's averaged to 67 . even though the clearly wrong information in the initial questions should have been irrelevant , it still affected the students ' estimates . this is an example of the anchoring effect , and it 's often used in marketing and negotiations to raise the prices that people are willing to pay . so , if heuristics lead to all these wrong decisions , why do we even have them ? well , because they can be quite effective . for most of human history , survival depended on making quick decisions with limited information . when there 's no time to logically analyze all the possibilities , heuristics can sometimes save our lives . but today 's environment requires far more complex decision-making , and these decisions are more biased by unconscious factors than we think , affecting everything from health and education to finance and criminal justice . we ca n't just shut off our brain 's heuristics , but we can learn to be aware of them . when you come to a situation involving numbers , probability , or multiple details , pause for a second and consider that the intuitive answer might not be the right one after all .

but studies have found that for many people , the negative psychological impact we feel from losing something is about twice as strong as the positive impact of gaining the same thing . loss aversion is one cognitive bias that arises from heuristics , problem-solving approaches based on previous experience and intuition rather than careful analysis . and these mental shortcuts can lead to irrational decisions , not like falling in love or bungee jumping off a cliff , but logical fallacies that can easily be proven wrong .

heuristics can be advantageous from an evolutionally perspective . can you think of a scenario in which being guided by an intuitive decision is actually better than a careful analysis of the situation ?

if you 've had surgery , you might remember starting to count backwards from ten , nine , eight , and then waking up with the surgery already over before you even got to five . and it might seem like you were asleep , but you were n't . you were under anesthesia , which is much more complicated . you were unconscious , but you also could n't move , form memories , or , hopefully , feel pain . without being able to block all those processes at once , many surgeries would be way too traumatic to perform . ancient medical texts from egypt , asia and the middle east all describe early anesthetics containing things like opium poppy , mandrake fruit , and alcohol . today , anesthesiologists often combine regional , inhalational and intravenous agents to get the right balance for a surgery . regional anesthesia blocks pain signals from a specific part of the body from getting to the brain . pain and other messages travel through the nervous system as electrical impulses . regional anesthetics work by setting up and electrical baracade . they bind to the proteins in neurons ' cell membranes that let charged particles in and out , and lock out positively charged particles . one compound that does this is cocaine , whose painkilling effects were discovered by accident when an ophthalmology intern got some on his tongue . it 's still occasionally used as an anesthetic , but many of the more common regional anesthetics have a similar chemical structure and work the same way . but for major surgeries where you need to be unconscious , you 'll want something that acts on the entire nervous system , including the brain . that 's what inhalational anesthetics do . in western medicine , diethyl ether was the first common one . it was best known as a recreational drug until doctors started to realize that people sometimes did n't notice injuries they received under the influence . in the 1840s , they started sedating patients with ether during dental extractions and surgeries . nitrous oxide became popular in the decades that followed and is still used today . although ether derivatives , like sevoflurane , are more common . inhalational anesthesia is usually supplemented with intravenous anesthesia , which was developed in the 1870s . common intravenous agents include sedatives , like propofol , which induce unconsciousness , and opioids , like fentanyl , which reduce pain . these general anesthetics also seem to work by affecting electrical signals in the nervous system . normally , the brain 's electrical signals are a chaotic chorus as different parts of the brain communicate with each other . that connectivity keeps you awake and aware . but as someone becomes anesthetized , those signals become calmer and more organized , suggesting that different parts of the brain are n't talking to each other anymore . there 's a lot we still do n't know about exactly how this happens . several common anesthetics bind to the gaba-a receptor in the brain 's neurons . they hold the gateway open , letting negatively charged particles flow into the cell . negative charge builds up and acts like a log jam , keeping the neuron from transmitting electrical signals . the nervous system has lots of these gated channels , controlling pathways for movement , memory , and consciousness . most anesthetics probably act on more than one , and they do n't act on just the nervous system . many anesthetics also affect the heart , lungs , and other vital organs . just like early anesthetics , which included familiar poisons like hemlock and aconite , modern drugs can have serious side effects . so an anesthesiologist has to mix just the right balance of drugs to create all the features of anesthesia , while carefully monitoring the patient 's vital signs , and adjusting the drug mixture as needed . anesthesia is complicated , but figuring out how to use it allowed for the development of new and better surgical techniques . surgeons could learn how to routinely and safely perform c-sections , reopen blocked arteries , replace damaged livers and kidneys , and many other life-saving operations . and each year , new anesthesia techniques are developed that will ensure more and more patients survive the trauma of surgery .

they bind to the proteins in neurons ' cell membranes that let charged particles in and out , and lock out positively charged particles . one compound that does this is cocaine , whose painkilling effects were discovered by accident when an ophthalmology intern got some on his tongue . it 's still occasionally used as an anesthetic , but many of the more common regional anesthetics have a similar chemical structure and work the same way .

the regional anesthetic , lidocaine , doctors commonly use today was first discovered by a ( future ) ophthalmology resident when he accidentally tasted this illegal narcotic on his tongue after his friend , sigmund freud , gave him a package to help elevate his mood .

think about all the things that need to happen for a human settlement to thrive : obtaining food , building shelter , raising children and more . there needs to be a way to divide resources , organize major efforts and distribute labor efficiently . now imagine having to do this without any sort of planning or higher level communication . welcome to the ant colony . ants have some of the most complex social organization in the animal kingdom , living in structured colonies containing different types of members who perform specific roles . but although this may sound similar to some human societies , this organization does n't arise from any higher level decisions , but is part of a biologically programmed cycle . in many species , all the winged males and winged virgin queens from all the nearby colonies in the population each leave from their different nests and meet at a central place to mate , using pheromones to guide each other to a breeding ground . after mating , the males die off , while females try to establish a new colony . the few that are successful settle down in a suitable spot , lose their wings , and begin laying eggs , selectively fertilizing some using stored sperm they 've saved up from mating . fertilized eggs grow into female workers who care for the queen and her eggs . they will then defend the colony and forage for food , while unfertilized eggs grow into males whose only job is to wait until they are ready to leave the nest and reproduce , beginning the cycle again . so how do worker ants decide what to do and when ? well , they do n't really . although they have no methods of intentional communication , individual ants do interact with one another through touch , sound and chemical signals . these stimuli accomplish many things from serving as an alarm to other ants if one is killed , to signaling when a queen is nearing the end of her reproductive life . but one of the most impressive collective capabilities of an ant colony is to thoroughly and efficiently explore large areas without any predetermined plan . most species of ants have little or no sense of sight and can only smell things in their vicinity . combined with their lack of high level coordination , this would seem to make them terrible explorers , but there is an amazingly simple way that ants maximize their searching efficiency ; by changing their movement patterns based on individual interactions . when two ants meet , they sense each other by touching antennae . if there are many ants in a small area this will happen more often causing them to respond by moving in more convoluted , random paths in order to search more thoroughly . but in a larger area , with less ants , where such meetings happen less often , they can walk in straight lines to cover more ground . while exploring their environment in this way , an ant may come across any number of things , from threats or enemies , to alternate nesting sites . and some species have another capability known as recruitment . when one of these ants happens to find food , it will return with it , marking its path with a chemical scent . other ants will then follow this pheromone trail , renewing it each time they manage to find food and return . once the food in that spot is depleted , the ants stop marking their return . the scent dissipates and ants are no longer attracted to that path . these seemingly crude methods of search and retrieval are , in fact , so useful that they are applied in computer models to obtain optimal solutions from decentralized elements , working randomly and exchanging simple information . this has many theoretical and practical applications , from solving the famous traveling salesman problem , to scheduling computing tasks and optimizing internet searches , to enabling groups of robots to search a minefield or a burning building collectively , without any central control . but you can observe these fascinatingly simple , yet effective , processes directly through some simple experiments , by allowing ants to enter empty spaces of various sizes and paying attention to their behavior . ants may not be able to vote , hold meetings or even make any plans , but we humans may still be able to learn something from the way that such simple creatures are able to function so effectively in such complex ways .

now imagine having to do this without any sort of planning or higher level communication . welcome to the ant colony . ants have some of the most complex social organization in the animal kingdom , living in structured colonies containing different types of members who perform specific roles .

who tells an ant what to do ?

to understand what 's happening with greece and the eurozone , think about a dinner party . if you 're cooking just for yourself and your spouse , it 's easy : you make something you both like . but if you 've got guests , things get harder . if you need to accommodate a vegetarian , and someone who is gluten free , and someone with a soy allergy , your options get really limited . and that 's the problem with europe 's idea of having a whole bunch of countries all use the same currency . so greece 's economy is in a disaster . a quarter of the population is unemployed , and it has this very high debt burden . normally , if you 've got really high unemployment , what happens is that a country makes its currency cheaper by printing extra money . that makes its products cheaper on world markets , it makes it a more attractive tourist destination , and it means that foreign investors can get great bargains . but if unemployment is really low , a country likes to have an expensive currency . that increases people 's purchasing power and it keeps prices down . and in europe , you have a bunch of economies that are really different . a price of euros that 's appropriate for greece , where they have a 25 % unemployment rate is way too low for germany , where the unemployment rate is below 5 % . and greece 's problem is that it 's small , poor , and geographically isolated from the rest of the eurozone . it 's like the only vegetarian at a barbeque , except when it comes to currencies , there 's no side dishes . and so there 's plenty of specific decisions we can second-guess , plenty of things greece did and various banks did that we can question , but fundamentally having all these countries come to a dinner party with only one dish on the menu was a mistake . the euro was a project that europe set about on for really political reasons . it was a symbol of their determination to have peace on the continent , but they did n't really take the economics of it seriously . so greece joins the euro in 2001 and initially , it works out great for greece because all of a sudden everyone was like 'yeah , sure , let 's lend them money . ' so they borrowed lots and lots and lots of euros , except that did n't change the fact that their economy is a lot weaker than some of the other european countries . so to really work , you would need a much , much , much closer union , where you had big financial transfers coming from the richer places to the poorer places all the time . in the united states , the poor states like kentucky , mississippi , alabama , they 're constantly getting money from the richer states like massachusetts , california , new york , through the welfare system , through social security , through medicare , through medicaid . and you know , people may complain about this or that program , but we do n't dispute the idea that it 's all one country so money is going to circulate around . europeans , you know , they just do n't feel that way . germans are willing to support poor german people , but they do n't want to support greek people with their tax dollars . so they 're kind of like half-way integrated in a way that does n't really work .

that makes its products cheaper on world markets , it makes it a more attractive tourist destination , and it means that foreign investors can get great bargains . but if unemployment is really low , a country likes to have an expensive currency . that increases people 's purchasing power and it keeps prices down .

the main objective of having a single currency in eu was to :

translator : andrea mcdonough reviewer : bedirhan cinar as any current or past geometry student knows , the father of geometry was euclid , a greek mathematician who lived in alexandria , egypt , around 300 b.c.e . euclid is known as the author of a singularly influential work known as `` elements . '' you think your math book is long ? euclid 's `` elements '' is 13 volumes full of just geometry . in `` elements , '' euclid structured and supplemented the work of many mathematicians that came before him , such as pythagoras , eudoxus , hippocrates and others . euclid laid it all out as a logical system of proof built up from a set of definitions , common notions , and his five famous postulates . four of these postulates are very simple and straightforward , two points determine a line , for example . the fifth one , however , is the seed that grows our story . this fifth mysterious postulate is known simply as the parallel postulate . you see , unlike the first four , the fifth postulate is worded in a very convoluted way . euclid 's version states that , `` if a line falls on two other lines so that the measure of the two interior angles on the same side of the transversal add up to less than two right angles , then the lines eventually intersect on that side , and therefore are not parallel . '' wow , that is a mouthful ! here 's the simpler , more familiar version : `` in a plane , through any point not on a given line , only one new line can be drawn that 's parallel to the original one . '' many mathematicians over the centuries tried to prove the parallel postulate from the other four , but were n't able to do so . in the process , they began looking at what would happen logically if the fifth postulate were actually not true . some of the greatest minds in the history of mathematics ask this question , people like ibn al-haytham , omar khayyam , nasir al-din al-tusi , giovanni saccheri , jános bolyai , carl gauss , and nikolai lobachevsky . they all experimented with negating the parallel postulate , only to discover that this gave rise to entire alternative geometries . these geometries became collectively known as non-euclidean geometries . we 'll leave the details of these different geometries for another lesson . the main difference depends on the curvature of the surface upon which the lines are constructed . turns out euclid did not tell us the entire story in `` elements , '' and merely described one possible way to look at the universe . it all depends on the context of what you 're looking at . flat surfaces behave one way , while positively and negatively curved surfaces display very different characteristics . at first these alternative geometries seemed strange , but were soon found to be equally adept at describing the world around us . navigating our planet requires elliptical geometry while the much of the art of m.c . escher displays hyperbolic geometry . albert einstein used non-euclidean geometry as well to describe how space-time becomes warped in the presence of matter , as part of his general theory of relativity . the big mystery is whether euclid had any inkling of the existence of these different geometries when he wrote his postulate . we may never know , but it 's hard to believe he had no idea whatsoever of their nature , being the great intellect that he was and understanding the field as thoroughly as he did . maybe he did know and he wrote the postulate in such a way as to leave curious minds after him to flush out the details . if so , he 's probably pleased . these discoveries could never have been made without gifted , progressive thinkers able to suspend their preconceived notions and think outside of what they 've been taught . we , too , must be willing at times to put aside our preconceived notions and physical experiences and look at the larger picture , or we risk not seeing the rest of the story .

the fifth one , however , is the seed that grows our story . this fifth mysterious postulate is known simply as the parallel postulate . you see , unlike the first four , the fifth postulate is worded in a very convoluted way .

which of euclid 's postulates is known as the parallel postulate ?

you and a fellow castaway are stranded on a desert island playing dice for the last banana . you 've agreed on these rules : you 'll roll two dice , and if the biggest number is one , two , three or four , player one wins . if the biggest number is five or six , player two wins . let 's try twice more . here , player one wins , and here it 's player two . so who do you want to be ? at first glance , it may seem like player one has the advantage since she 'll win if any one of four numbers is the highest , but actually , player two has an approximately 56 % chance of winning each match . one way to see that is to list all the possible combinations you could get by rolling two dice , and then count up the ones that each player wins . these are the possibilities for the yellow die . these are the possibilities for the blue die . each cell in the chart shows a possible combination when you roll both dice . if you roll a four and then a five , we 'll mark a player two victory in this cell . a three and a one gives player one a victory here . there are 36 possible combinations , each with exactly the same chance of happening . mathematicians call these equiprobable events . now we can see why the first glance was wrong . even though player one has four winning numbers , and player two only has two , the chance of each number being the greatest is not the same . there is only a one in 36 chance that one will be the highest number . but there 's an 11 in 36 chance that six will be the highest . so if any of these combinations are rolled , player one will win . and if any of these combinations are rolled , player two will win . out of the 36 possible combinations , 16 give the victory to player one , and 20 give player two the win . you could think about it this way , too . the only way player one can win is if both dice show a one , two , three or four . a five or six would mean a win for player two . the chance of one die showing one , two , three or four is four out of six . the result of each die roll is independent from the other . and you can calculate the joint probability of independent events by multiplying their probabilities . so the chance of getting a one , two , three or four on both dice is 4/6 times 4/6 , or 16/36 . because someone has to win , the chance of player two winning is 36/36 minus 16/36 , or 20/36 . those are the exact same probabilities we got by making our table . but this does n't mean that player two will win , or even that if you played 36 games as player two , you 'd win 20 of them . that 's why events like dice rolling are called random . even though you can calculate the theoretical probability of each outcome , you might not get the expected results if you examine just a few events . but if you repeat those random events many , many , many times , the frequency of a specific outcome , like a player two win , will approach its theoretical probability , that value we got by writing down all the possibilities and counting up the ones for each outcome . so , if you sat on that desert island playing dice forever , player two would eventually win 56 % of the games , and player one would win 44 % . but by then , of course , the banana would be long gone .

but if you repeat those random events many , many , many times , the frequency of a specific outcome , like a player two win , will approach its theoretical probability , that value we got by writing down all the possibilities and counting up the ones for each outcome . so , if you sat on that desert island playing dice forever , player two would eventually win 56 % of the games , and player one would win 44 % . but by then , of course , the banana would be long gone .

now that you learned how to calculate the probabilities of each player win the “ maximum game ” in the video , let 's look at the probabilities of another game . this is how it works : we roll two dice and calculate the multiplication of the two numbers we rolled . -- if it is a multiple of 6 , i win -- if it is not a multiple of 6 , you win . here is an example : if you get 3 and 4 , the multiplication is 12. twelve is a multiple of 6 , so i win ! 1. which player would win if you get 2 and 5 in the dice ? me or you ? 2. which player would win if you get 4 and 2 in the dice ? 3. which player would win if you get 1 and 6 in the dice ?

it starts with a tickle in your throat that becomes a cough . your muscles begin to ache , you grow irritable , and you lose your appetite . it 's official : you 've got the flu . it 's logical to assume that this miserable medley of symptoms is the result of the infection coursing through your body , but is that really the case ? what 's actually making you feel sick ? what if your body itself was driving this vicious onslaught ? you first get ill when a pathogen like the flu virus gets into your system , infecting and killing your cells . but this unwelcome intrusion has another effect : it alerts your body 's immune system to your plight . as soon as it becomes aware of infection , your body leaps to your defense . cells called macrophages charge in as the first line of attack , searching for and destroying the viruses and infected cells . afterwards , the macrophages release protein molecules called cytokines whose job is to recruit and organize more virus-busting cells from your immune system . if this coordinated effort is strong enough , it 'll wipe out the infection before you even notice it . but that 's just your body setting the scene for some real action . in some cases , viruses spread further , even into the blood and vital organs . to avoid this sometimes dangerous fate , your immune system must launch a stronger attack , coordinating its activity with the brain . that 's where those unpleasant symptoms come in , starting with the surging temperature , aches and pains , and sleepiness . so why do we experience this ? when the immune system is under serious attack , it secretes more cytokines , which trigger two responses . first , the vagus nerve , which runs through the body into the brain , quickly transmits the information to the brain stem , passing near an important area of pain processing . second , cytokines travel through the body to the hypothalamus , the part of the brain responsible for controlling temperature , thirst , hunger , and sleep , among other things . when it receives this message , the hypothalamus produces another molecule called prostaglandin e2 , which gears it up for war . the hypothalamus sends signals that instruct your muscles to contract and causes a rise in body temperature . it also makes you sleepy , and you lose your appetite and thirst . but what 's the point of all of these unpleasant symptoms ? well , we 're not yet sure , but some theorize that they aid in recovery . the rise in temperature can slow bacteria and help your immune system destroy pathogens . sleep lets your body channel more energy towards fighting infection . when you stop eating , your liver can take up much of the iron in your blood , and since iron is essential for bacterial survival , that effectively starves them . your reduced thirst makes you mildly dehydrated , diminishing transmission through sneezes , coughs , vomit , or diarrhea . though it 's worth noting that if you do n't drink enough water , that dehydration can become dangerous . even the body 's aches make you more sensitive , drawing attention to infected cuts that might be worsening , or even causing your condition . in addition to physical symptoms , sickness can also make you irritable , sad , and confused . that 's because cytokines and prostaglandin can reach even higher structures in your brain , disrupting the activity of neurotransmitters , like glutamate , endorphins , serotonin , and dopamine . this affects areas like the limbic system , which oversees emotions , and your cerebral cortex , which is involved in reasoning . so it 's actually the body 's own immune response that causes much of the discomfort you feel every time you get ill . unfortunately , it does n't always work perfectly . most notably , millions of people worldwide suffer from autoimmune diseases , in which the immune system treats normal bodily cues as threats , so the body attacks itself . but for the majority of the human race , millions of years of evolution have fine-tuned the immune system so that it works for , rather than against us . the symptoms of our illnesses are annoying , but collectively , they signify an ancient process that will continue barricading our bodies against the outside world for centuries to come .

in some cases , viruses spread further , even into the blood and vital organs . to avoid this sometimes dangerous fate , your immune system must launch a stronger attack , coordinating its activity with the brain . that 's where those unpleasant symptoms come in , starting with the surging temperature , aches and pains , and sleepiness .

one way in which the immune system communicates with the brain is by :

james joyce 's `` ulysses '' is widely considered to be both a literary masterpiece and one of the hardest works of literature to read . it inspires such devotion that once a year on a day called bloomsday , thousands of people all over the world dress up like the characters , take to the streets , and read the book aloud . and some even make a pilgrimage to dublin just to visit the places so vividly depicted in joyce 's opus . so what is it about this famously difficult novel that inspires so many people ? there 's no one simple answer to that question , but there are a few remarkable things about the book that keep people coming back . the plot , which transpires over the course of a single day , is a story of three characters : stephen dedalus , reprised from joyce 's earlier novel , `` a portrait of the artist as a young man '' ; leopold bloom , a half-jewish advertising canvasser for a dublin newspaper ; and bloom 's wife molly , who is about to embark on an affair . stephen is depressed because of his mother 's recent death . meanwhile , bloom wanders throughout the city . he goes to a funeral , his work , a pub , and so on , avoiding going home because molly is about to begin her affair . where it really starts to get interesting , though , is how the story 's told . each chapter is written in a different style . 15 is a play , 13 is like a cheesy romance novel , 12 is a story with bizarre , exaggerated interruptions , 11 uses techniques , like onomatopoeia , repetitions , and alliteration to imitate music , and 14 reproduces the evolution of english literary prose style , from its beginnings in anglo-saxon right up to the 20th century . that all culminates in the final chapter which follows molly 's stream of consciousness as it spools out in just eight long paragraphs with almost no punctuation . the range of styles joyce uses in `` ulysses '' is one of the things that makes it so difficult , but it also helps make it enjoyable . and it 's one of the reasons that the book has held up as one of the key texts of literary modernism , a movement characterized by overturning traditional modes of writing . joyce fills his narrative gymnastic routines with some of the most imaginative use of language you 'll find anywhere . take , for instance , `` the figure seated on a large boulder at the foot of a round tower was that of a broadshouldered deepchested stronglimbed frankeyed redhaired freelyfreckled shaggybearded widemouthed largenosed longheaded deepvoiced barekneed brawnyhanded hairlegged ruddyfaced sinewyarmed hero . '' here , joyce exaggerates the description of a mangy old man in a pub to make him seem like an improbably gigantesque hero . it 's true that some sections are impenetrably dense at first glance , but it 's up to the reader to let their eyes skim over them or break out a shovel and dig in . and once you start excavating the text , you 'll find the book to be an encyclopedic treasure trove . it 's filled with all manner of references and allusions from medieval philosophy to the symbolism of tattoos , and from dante to dublin slang . as suggested by the title , some of these allusions revolve around homer 's `` odyssey . '' each chapter is named after a character or episode from the `` odyssey , '' but the literary references are often coy , debatable , sarcastic , or disguised . for example , homer 's odysseus , after an epic 20-year-long journey , returns home to ithaca and reunites with his faithful wife . in contrast , joyce 's bloom wanders around dublin for a day and returns home to his unfaithful wife . it 's a very funny book . it has highbrow intellectual humor , if you have the patience to track down joyce 's references , and more lowbrow dirty jokes . those , and other sexual references , were too much for some . in the u.s. , the book was put on trial , banned , and censored before it had even been completed because it was originally published as a serial novel . readers of `` ulysses '' are n't just led through a variety of literary styles . they 're also given a rich and shockingly accurate tour of a specific place at a time : dublin in 1904 . joyce claimed that if dublin were to be destroyed , it could be recreated from the pages of this book . while such a claim is not exactly true , it does show the great care that joyce took in precisely representing details , both large and small , of his home city . no small feat considering he wrote the entire novel while living outside of his native ireland . it 's a testament to joyce 's genius that `` ulysses '' is a difficult book . some people find it impenetrable without a full book of annotations to help them understand what joyce is even talking about . but there 's a lot of joy to be found in reading it , more than just unpacking allusions and solving puzzles . and if it 's difficult , or frustrating , or funny , that 's because life is all that , and more . responding to some criticism of `` ulysses , '' and there was a lot when it was first published , joyce said that if `` ulysses '' is n't worth reading , then life is n't worth living .

stephen is depressed because of his mother 's recent death . meanwhile , bloom wanders throughout the city . he goes to a funeral , his work , a pub , and so on , avoiding going home because molly is about to begin her affair .

what is leopold bloom ’ s job ?

in 1879 , amateur archaeologist marcelino sanz de sautuola and his young daughter maria explored a dark cave in northern spain . when maria wondered off by herself , she made an amazing discovery . they were standing inside a site of ancient art , the walls and roofs decorated with prehistoric paintings and engravings , ranging from 19,000 to 35,000 years old . similar marks of our ancestors have been preserved in caves all over the world . the oldest we 've found were made up to 40,000 years ago . what do these images tell us about the ancient human mind and the lives of their creators ? these early artists mixed minerals , clay , charcoal , and ochre with spit or animal fat to create paint . they drew with their hands and tools , like pads of moss , twigs , bones , and hair . in many instances , their images follow the contours of the cave to create depth and shade . the most common depictions are of geometric shapes , followed by large mammals , like bison , horses , mammoths , deer , and boars . human figures appear rarely , as well as occasional hand prints . some have theorized that these artworks are the creation of hunters , or of holy men in trance-like states . and we 've found examples created by men , women , and even children . and why did they create this art ? perhaps they were documenting what they knew about the natural world , like modern scientists , or marking their tribal territory . maybe the images were the culmination of sacred hunting rituals or spiritual journeys . or could they be art for art 's sake , the sheer joy and fulfillment of creation ? as with many unsolved mysteries of the ancient world , we may never know for sure , barring the invention of a time machine , that is . but while the answers remain elusive , these images are our earliest proof of human communication , testifying to the human capacity for creativity thousands of years before writing . they are a distinct visual language that imagines the world outside the self , just like modern art forms , from graffiti and painting to animated virtual-reality caves .

when maria wondered off by herself , she made an amazing discovery . they were standing inside a site of ancient art , the walls and roofs decorated with prehistoric paintings and engravings , ranging from 19,000 to 35,000 years old . similar marks of our ancestors have been preserved in caves all over the world .

what sort of historical record do cave paintings provide ? how can we use them as an artifact in thinking through the earliest years of civilization ?

imagine , for a second , a duck teaching a french class , a ping-pong match in orbit around a black hole , a dolphin balancing a pineapple . you probably have n't actually seen any of these things , but you could imagine them instantly . how does your brain produce an image of something you 've never seen ? that may not seem hard , but that 's only because we 're so used to doing it . it turns out that this is actually a complex problem that requires sophisticated coordination inside your brain . that 's because to create these new , weird images , your brain takes familiar pieces and assembles them in new ways , like a collage made from fragments of photos . the brain has to juggle a sea of thousands of electrical signals getting them all to their destination at precisely the right time . when you look at an object , thousands of neurons in your posterior cortex fire . these neurons encode various characteristics of the object : spiky , fruit , brown , green , and yellow . this synchronous firing strengthens the connections between that set of neurons , linking them together into what 's known as a neuronal ensemble , in this case the one for pineapple . in neuroscience , this is called the hebbian principle , neurons that fire together wire together . if you try to imagine a pineapple later , the whole ensemble will light up , assembling a complete mental image . dolphins are encoded by a different neuronal ensemble . in fact , every object that you 've seen is encoded by a neuronal ensemble associated with it , the neurons wired together by that synchronized firing . but this principle does n't explain the infinite number of objects that we can conjure up in our imaginations without ever seeing them . the neuronal ensemble for a dolphin balancing a pineapple does n't exist . so how come you can imagine it anyway ? one hypothesis , called the mental synthesis theory , says that , again , timing is key . if the neuronal ensembles for the dolphin and pineapple are activated at the same time , we can perceive the two separate objects as a single image . but something in your brain has to coordinate that firing . one plausible candidate is the prefrontal cortex , which is involved in all complex cognitive functions . prefrontal cortex neurons are connected to the posterior cortex by long , spindly cell extensions called neural fibers . the mental synthesis theory proposes that like a puppeteer pulling the strings , the prefrontal cortex neurons send electrical signals down these neural fibers to multiple ensembles in the posterior cortex . this activates them in unison . if the neuronal ensembles are turned on at the same time , you experience the composite image just as if you 'd actually seen it . this conscious purposeful synchronization of different neuronal ensembles by the prefrontal cortex is called mental synthesis . in order for mental sythesis to work , signals would have to arrive at both neuronal ensembles at the same time . the problem is that some neurons are much farther away from the prefrontal cortex than others . if the signals travel down both fibers at the same rate , they 'd arrive out of sync . you ca n't change the length of the connections , but your brain , especially as it develops in childhood , does have a way to change the conduction velocity . neural fibers are wrapped in a fatty substance called myelin . myelin is an insulator and speeds up the electrical signals zipping down the nerve fiber . some neural fibers have as many as 100 layers of myelin . others only have a few . and fibers with thicker layers of myelin can conduct signals 100 times faster or more than those with thinner ones . some scientists now think that this difference in myelination could be the key to uniform conduction time in the brain , and consequently , to our mental synthesis ability . a lot of this myelination happens in childhood , so from an early age , our vibrant imaginations may have a lot to do with building up brains whose carefully myelinated connections can craft creative symphonies throughout our lives .

if you try to imagine a pineapple later , the whole ensemble will light up , assembling a complete mental image . dolphins are encoded by a different neuronal ensemble . in fact , every object that you 've seen is encoded by a neuronal ensemble associated with it , the neurons wired together by that synchronized firing .

which of the following statements about a neuronal ensemble is untrue ?

it begins with a bit of discomfort and soon becomes a pressing sensation that 's impossible to ignore . finally , it 's all you can think about , and out of sheer desperation , you go on a hunt for a bathroom until `` ahh . '' humans should urinate at least four to six times a day , but occasionally , the pressures of modern life forces us to clench and hold it in . how bad is this habit , and how long can our bodies withstand it ? the answers lie in the workings of the bladder , an oval pouch that sits inside the pelvis . surrounding this structure are several other organs that together make up the whole urinary system . two kidneys , two ureters , two urethral sphincters , and a urethra . constantly trickling down from the kidneys is the yellowish liquid known as urine . the kidneys make urine from a mix of water and the body 's waste products , funneling the unwanted fluid into two muscular tubes called ureters . these carry it downward into the hollow organ known as the bladder . this organ 's muscular wall is made of tissue called detrusor muscle which relaxes as the bladder fills allowing it to inflate like a balloon . as the bladder gets full , the detrusor contracts . the internal urethral sphincter automatically and involuntarily opens , and the urine is released . whooshing downwards , the fluid enters the urethra and stops short at the external urethral sphincter . this works like a tap . when you want to delay urinating , you keep the sphincter closed . when you want to release it , you can voluntarily open the flood gates . but how do you sense your bladder 's fullness so you know when to pee ? inside the layers of detrusor muscles are millions of stretch receptors that get triggered as the bladder fills . they send signals along your nerves to the sacral region in your spinal cord . a reflex signal travels back to your bladder , making the detrusor muscle contract slightly and increasing the bladder 's pressure so you 're aware that it 's filling up . simultaneously , the internal urethral sphincter opens . this is called the micturition reflex . the brain can counter it if it 's not a good time to urinate by sending another signal to contract the external urethral sphincter . with about 150 to 200 milliliters of urine inside of it , the bladder 's muscular wall is stretched enough for you to sense that there 's urine within . at about 400 to 500 milliliters , the pressure becomes uncomfortable . the bladder can go on stretching , but only to a point . above 1,000 milliliters , it may burst . most people would lose bladder control before this happens , but in very rare cases , such as when as a person ca n't sense the need to urinate , the pouch can rupture painfully requiring surgery to fix . but under normal circumstances , your decision to urinate stops the brain 's signal to the external urethral sphincter , causing it to relax and the bladder to empty . the external urethral sphincter is one of the muscles of the pelvic floor , and it provides support to the urethra and bladder neck . it 's lucky we have these pelvic floor muscles because placing pressure on the system by coughing , sneezing , laughing , or jumping could cause bladder leakage . instead , the pelvic floor muscles keep the region sealed until you 're ready to go . but holding it in for too long , forcing out your urine too fast , or urinating without proper physical support may over time weaken or overwork that muscular sling . that can lead to an overactive pelvic floor , bladder pain , urgency , or urinary incontinence . so in the interest of long-term health , it 's not a great habit to hold your pee . but in the short term , at least , your body and brain have got you covered , so you can conveniently choose your moment of sweet release .

but holding it in for too long , forcing out your urine too fast , or urinating without proper physical support may over time weaken or overwork that muscular sling . that can lead to an overactive pelvic floor , bladder pain , urgency , or urinary incontinence . so in the interest of long-term health , it 's not a great habit to hold your pee .

which part of the urinary system is also part of the pelvic floor muscles ?

when i was in fourth grade , my teacher said to us one day : `` there are as many even numbers as there are numbers . '' `` really ? `` , i thought . well , yeah , there are infinitely many of both , so i suppose there are the same number of them . but even numbers are only part of the whole numbers , all the odd numbers are left over , so there 's got to be more whole numbers than even numbers , right ? to see what my teacher was getting at , let 's first think about what it means for two sets to be the same size . what do i mean when i say i have the same number of fingers on my right hand as i do on left hand ? of course , i have five fingers on each , but it 's actually simpler than that . i do n't have to count , i only need to see that i can match them up , one to one . in fact , we think that some ancient people who spoke languages that did n't have words for numbers greater than three used this sort of magic . for instance , if you let your sheep out of a pen to graze , you can keep track of how many went out by setting aside a stone for each one , and putting those stones back one by one when the sheep return , so you know if any are missing without really counting . as another example of matching being more fundamental than counting , if i 'm speaking to a packed auditorium , where every seat is taken and no one is standing , i know that there are the same number of chairs as people in the audience , even though i do n't know how many there are of either . so , what we really mean when we say that two sets are the same size is that the elements in those sets can be matched up one by one in some way . my fourth grade teacher showed us the whole numbers laid out in a row , and below each we have its double . as you can see , the bottom row contains all the even numbers , and we have a one-to-one match . that is , there are as many even numbers as there are numbers . but what still bothers us is our distress over the fact that even numbers seem to be only part of the whole numbers . but does this convince you that i do n't have the same number of fingers on my right hand as i do on my left ? of course not . it does n't matter if you try to match the elements in some way and it does n't work , that does n't convince us of anything . if you can find one way in which the elements of two sets do match up , then we say those two sets have the same number of elements . can you make a list of all the fractions ? this might be hard , there are a lot of fractions ! and it 's not obvious what to put first , or how to be sure all of them are on the list . nevertheless , there is a very clever way that we can make a list of all the fractions . this was first done by georg cantor , in the late eighteen hundreds . first , we put all the fractions into a grid . they 're all there . for instance , you can find , say , 117/243 , in the 117th row and 223rd column . now we make a list out of this by starting at the upper left and sweeping back and forth diagonally , skipping over any fraction , like 2/2 , that represents the same number as one the we 've already picked . we get a list of all the fractions , which means we 've created a one-to-one match between the whole numbers and the fractions , despite the fact that we thought maybe there ought to be more fractions . ok , here 's where it gets really interesting . you may know that not all real numbers -- that is , not all the numbers on a number line -- are fractions . the square root of two and pi , for instance . any number like this is called irrational . not because it 's crazy , or anything , but because the fractions are ratios of whole numbers , and so are called rationals ; meaning the rest are non-rational , that is , irrational . irrationals are represented by infinite , non-repeating decimals . so , can we make a one-to-one match between the whole numbers and the set of all the decimals , both the rationals and the irrationals ? that is , can we make a list of all the decimal numbers ? candor showed that you ca n't . not merely that we do n't know how , but that it ca n't be done . look , suppose you claim you have made a list of all the decimals . i 'm going to show you that you did n't succeed , by producing a decimal that is not on your list . i 'll construct my decimal one place at a time . for the first decimal place of my number , i 'll look at the first decimal place of your first number . if it 's a one , i 'll make mine a two ; otherwise i 'll make mine a one . for the second place of my number , i 'll look at the second place of your second number . again , if yours is a one , i 'll make mine a two , and otherwise i 'll make mine a one . see how this is going ? the decimal i 've produced ca n't be on your list . why ? could it be , say , your 143rd number ? no , because the 143rd place of my decimal is different from the 143rd place of your 143rd number . i made it that way . your list is incomplete . it does n't contain my decimal number . and , no matter what list you give me , i can do the same thing , and produce a decimal that 's not on that list . so we 're faced with this astounding conclusion : the decimal numbers can not be put on a list . they represent a bigger infinity that the infinity of whole numbers . so , even though we 're familiar with only a few irrationals , like square root of two and pi , the infinity of irrationals is actually greater than the infinity of fractions . someone once said that the rationals -- the fractions -- are like the stars in the night sky . the irrationals are like the blackness . cantor also showed that , for any infinite set , forming a new set made of all the subsets of the original set represents a bigger infinity than that original set . this means that , once you have one infinity , you can always make a bigger one by making the set of all subsets of that first set . and then an even bigger one by making the set of all the subsets of that one . and so on . and so , there are an infinite number of infinities of different sizes . if these ideas make you uncomfortable , you are not alone . some of the greatest mathematicians of cantor 's day were very upset with this stuff . they tried to make this different infinities irrelevant , to make mathematics work without them somehow . cantor was even vilified personally , and it got so bad for him that he suffered severe depression , and spent the last half of his life in and out of mental institutions . but eventually , his ideas won out . today , they 're considered fundamental and magnificent . all research mathematicians accept these ideas , every college math major learns them , and i 've explained them to you in a few minutes . some day , perhaps , they 'll be common knowledge . there 's more . we just pointed out that the set of decimal numbers -- that is , the real numbers -- is a bigger infinity than the set of whole numbers . candor wondered whether there are infinities of different sizes between these two infinities . he did n't believe there were , but could n't prove it . candor 's conjecture became known as the continuum hypothesis . in 1900 , the great mathematician david hilbert listed the continuum hypothesis as the most important unsolved problem in mathematics . the 20th century saw a resolution of this problem , but in a completely unexpected , paradigm-shattering way . in the 1920s , kurt gödel showed that you can never prove that the continuum hypothesis is false . then , in the 1960s , paul j. cohen showed that you can never prove that the continuum hypothesis is true . taken together , these results mean that there are unanswerable questions in mathematics . a very stunning conclusion . mathematics is rightly considered the pinnacle of human reasoning , but we now know that even mathematics has its limitations . still , mathematics has some truly amazing things for us to think about .

in the 1920s , kurt gödel showed that you can never prove that the continuum hypothesis is false . then , in the 1960s , paul j. cohen showed that you can never prove that the continuum hypothesis is true . taken together , these results mean that there are unanswerable questions in mathematics .

the work of which two mathematicians showed that the question of whether or not the continuum hypothesis is true is an unanswerable question ?

how can you play a rubik 's cube ? not play with it , but play it like a piano ? that question does n't make a lot of sense at first , but an abstract mathematical field called group theory holds the answer , if you 'll bear with me . in math , a group is a particular collection of elements . that might be a set of integers , the face of a rubik 's cube , or anything , so long as they follow four specific rules , or axioms . axiom one : all group operations must be closed or restricted to only group elements . so in our square , for any operation you do , like turn it one way or the other , you 'll still wind up with an element of the group . axiom two : no matter where we put parentheses when we 're doing a single group operation , we still get the same result . in other words , if we turn our square right two times , then right once , that 's the same as once , then twice , or for numbers , one plus two is the same as two plus one . axiom three : for every operation , there 's an element of our group called the identity . when we apply it to any other element in our group , we still get that element . so for both turning the square and adding integers , our identity here is zero , not very exciting . axiom four : every group element has an element called its inverse also in the group . when the two are brought together using the group 's addition operation , they result in the identity element , zero , so they can be thought of as cancelling each other out . so that 's all well and good , but what 's the point of any of it ? well , when we get beyond these basic rules , some interesting properties emerge . for example , let 's expand our square back into a full-fledged rubik 's cube . this is still a group that satisfies all of our axioms , though now with considerably more elements and more operations . we can turn each row and column of each face . each position is called a permutation , and the more elements a group has , the more possible permutations there are . a rubik 's cube has more than 43 quintillion permutations , so trying to solve it randomly is n't going to work so well . however , using group theory we can analyze the cube and determine a sequence of permutations that will result in a solution . and , in fact , that 's exactly what most solvers do , even using a group theory notation indicating turns . and it 's not just good for puzzle solving . group theory is deeply embedded in music , as well . one way to visualize a chord is to write out all twelve musical notes and draw a square within them . we can start on any note , but let 's use c since it 's at the top . the resulting chord is called a diminished seventh chord . now this chord is a group whose elements are these four notes . the operation we can perform on it is to shift the bottom note to the top . in music that 's called an inversion , and it 's the equivalent of addition from earlier . each inversion changes the sound of the chord , but it never stops being a c diminished seventh . in other words , it satisfies axiom one . composers use inversions to manipulate a sequence of chords and avoid a blocky , awkward sounding progression . on a musical staff , an inversion looks like this . but we can also overlay it onto our square and get this . so , if you were to cover your entire rubik 's cube with notes such that every face of the solved cube is a harmonious chord , you could express the solution as a chord progression that gradually moves from discordance to harmony and play the rubik 's cube , if that 's your thing .

how can you play a rubik 's cube ? not play with it , but play it like a piano ?

there are _____ possible permutations in a rubik 's cube puzzle

gravity . it controls the universe . everything attracts everything else . ouch ! including you . ow ! in this final lesson , we 'll explore what gravity means for space-time , or rather what space-time means for gravity . until now , we 've been dealing with things moving at constant speeds , with straight world lines in space-time . but once you add gravity , if you measure a speed at one moment , then again a bit later , the speed may have changed . in other words , as i discovered , gravity causes acceleration , so we need the world line to look different from one moment to the next . as we saw in the last lesson , the correct way to tilt an object 's world line is using a lorentz transformation : einstein 's stretch and squash trick . so , to map out what gravity is doing to tom 's motion , we need to create a whole load of little patches of space-time , each transformed by different amounts . so that my world line is at a different angle in each one . and then , we 're ready to stitch everything together . we assemble a cozy quilt of space-time where world lines look curved . where the world lines join , the objects collide . by making these connections between the patches , a curvature gets built into space-time itself . but einstein 's true genius was to describe precisely how each patch is stretched and squashed according to nearby mass and energy . the mere presence of stuff curves the space-time , and curving space-time moves the stuff around . this is gravity , according to einstein . previously , isaac newton had explained gravity using the ideas of force and acceleration , without any wibbily wobbly space-time , and that did pretty well . but einstein 's theory does just slightly better at predicting , for example , the orbit of mercury around the sun , or the way that light rays are deflected by massive objects . more importantly , einstein 's theory predicts things that simply do n't exist in older theories where space , time and gravity were separate . the stitching can leave wrinkles in the space-time material . these are called gravitational waves , which should be detectable as tiny , repetitive , subtle squashes and stretches in space . so we 're building experiments to check if they are there . in the meantime , indirect evidence , most recently in the polarization patterns of light left over from the big bang , strongly suggest that they are . but despite einstein 's successes , when too much stuff gets concentrated in too small a space , like in a black hole , the curvature of space-time becomes so large , that his equations collapse . we need a new picture of space-time that incorporates quantum mechanics to unlock the secret at the heart of black holes . which means there 's plenty more to be discovered about space , time , and space-time in the future .

the mere presence of stuff curves the space-time , and curving space-time moves the stuff around . this is gravity , according to einstein . previously , isaac newton had explained gravity using the ideas of force and acceleration , without any wibbily wobbly space-time , and that did pretty well .

the expansion of the universe fits perfectly into einstein ’ s equations of gravity . but the expansion is speeding up , which is more surprising , because everything ought to be pulled back together . how can accelerated expansion be reconciled with einstein ’ s theory of gravity ? why do some people think it points to a new , better theory beyond einstein ’ s ?

hey , congratulations ! you 've just won the lottery , only the prize is n't cash or a luxury cruise . it 's a position in your country 's national legislature . and you are n't the only lucky winner . all of your fellow lawmakers were chosen in the same way . this might strike you as a strange way to run a government , let alone a democracy . elections are the epitome of democracy , right ? well , the ancient athenians who coined the word had another view . in fact , elections only played a small role in athenian democracy , with most offices filled by random lottery from a pool of citizen volunteers . unlike the representative democracies common today , where voters elect leaders to make laws and decisions on their behalf , 5th century bc athens was a direct democracy that encouraged wide participation through the principle of ho boulomenos , or anyone who wishes . this meant that any of its approximately 30,000 eligible citizens could attend the ecclesia , a general assembly meeting several times a month . in principle , any of the 6,000 or so who showed up at each session had the right to address their fellow citizens , propose a law , or bring a public lawsuit . of course , a crowd of 6,000 people trying to speak at the same time would not have made for effective government . so the athenian system also relied on a 500 member governing council called the boule , to set the agenda and evaluate proposals , in addition to hundreds of jurors and magistrates to handle legal matters . rather than being elected or appointed , the people in these positions were chosen by lot . this process of randomized selection is know as sortition . the only positions filled by elections were those recognized as requiring expertise , such as generals . but these were considered aristocratic , meaning rule by the best , as opposed to democracies , rule by the many . how did this system come to be ? well , democracy arose in athens after long periods of social and political tension marked by conflict among nobles . powers once restricted to elites , such as speaking in the assembly and having their votes counted , were expanded to ordinary citizens . and the ability of ordinary citizens to perform these tasks adequately became a central feature of the democratice ideology of athens . rather than a privilege , civic participation was the duty of all citizens , with sortition and strict term limits preventing governing classes or political parties from forming . by 21st century standards , athenian rule by the many excluded an awful lot of people . women , slaves and foreigners were denied full citizenship , and when we filter out those too young to serve , the pool of eligible athenians drops to only 10-20 % of the overall population . some ancient philosophers , including plato , disparaged this form of democracy as being anarchic and run by fools . but today the word has such positive associations , that vastly different regimes claim to embody it . at the same time , some share plato 's skepticism about the wisdom of crowds . many modern democracies reconcile this conflict by having citizens elect those they consider qualified to legislate on their behalf . but this poses its own problems , including the influence of wealth , and the emergence of professional politicians with different interests than their constituents . could reviving election by lottery lead to more effective government through a more diverse and representative group of legislatures ? or does modern political office , like athenian military command , require specialized knowledge and skills ? you probably should n't hold your breath to win a spot in your country 's government . but depending on where you live , you may still be selected to participate in a jury , a citizens ' assembly , or a deliberative poll , all examples of how the democratic principle behind sortition still survives today .

but this poses its own problems , including the influence of wealth , and the emergence of professional politicians with different interests than their constituents . could reviving election by lottery lead to more effective government through a more diverse and representative group of legislatures ? or does modern political office , like athenian military command , require specialized knowledge and skills ? you probably should n't hold your breath to win a spot in your country 's government .

which office was filled by election ?

translator : andrea mcdonough reviewer : bedirhan cinar feathers are some of the most remarkable things ever made by an animal . they are gorgeous in their complexity , delicate in their construction , and yet strong enough to hold a bird thousands of feet in the air . like all things in nature , feathers evolved over millions of years into their modern form . it could be hard to imagine how this could have happened . after all , what did the intermediate forms look like ? what good is half a wing , festooned with half-feathers ? thanks to science , we now know that birds are living dinosaurs . you can see the kinship in their skeletons . certain dinosaurs share some anatomical details with birds found in no other animals , such as wish bones . and in the late 1990s , paleontologists started digging up some compelling support for that idea : dinosaurs with bits of feathers still preserved on their bodies . since then , scientists have found dozens of species of dinosaurs with remnants of feathers . some were as small as pigeons , and some were the size of a school bus . if you look at how they are related on a family tree , the evolution of feathers does n't seem quite so impossible . the most distant feathered relatives of birds had straight feathers that looked like wires . then these wires split apart , producing simple branches . in many dinosaur lineages , these simple feathers evolved into more intricate ones , including some that we see today on birds . at the same time , the feathers spread across the bodies of dinosaurs , turning from sparse patches of fuzz into dense plumage , which even extended down to their legs . a few fossils even preserved some of the molecules that give feathers color . they reveal a beautiful range of colors : glossy , dark plumage , reminiscent of crows , alternating strips of black and white , or splashes of bright red . some dinosaurs had high crests on their heads , and others had long , dramatic tail feathers . now , none of these dinosaurs could use their feathers to fly - their arms were too short and the rest of their bodies were far too heavy . but , birds do n't just use feathers to fly . a woodcock uses feathers to blend in perfectly with its forest backdrop . an ostrich stretches its wings over its nest to shade its young . a peacock displays its magnificent tail feathers to attract peahens . feathers could have served these functions for dinosaurs too . exactly how feathered dinosaurs took flight is still a bit of a mystery . but if a small-feathered dinosaur flapped its arms as it ran up an incline , its feathers would have provided extra lift to help it run faster . this accident of physics might have led to the evolution of longer dinosaur arms , which would let them run faster and even leap short distances through the air . eventually , their arms stretched out into wings . only then , perhaps 50 million years after the first wiry feathers evolved , did feathers lift those dinosaurs into the sky .

but if a small-feathered dinosaur flapped its arms as it ran up an incline , its feathers would have provided extra lift to help it run faster . this accident of physics might have led to the evolution of longer dinosaur arms , which would let them run faster and even leap short distances through the air . eventually , their arms stretched out into wings .

what does zimmer mean by accident of physics ?

translator : andrea mcdonough reviewer : bedirhan cinar you believe that the sun is much larger than the earth , that the earth is a roughly spherical planet that rotates on its axis every 24 hours and it revolves around the sun once every 365 days . you believe that you were born on a particular date , that you were born to two human parents and that each of your human parents was born on an earlier date . you believe that other human beings have thoughts and feelings like you do and that you are not surrounded by humanoid robots . you believe all of these things and many more , not on the basis of direct observation , which ca n't , by itself , tell you very much about the relative size and motion of the sun and the earth , or about your own family history , or about what goes on in the minds of other humans . instead , these beliefs are mostly based on what you 've been told . without spoken and written testimonies , human beings could not pass on knowledge from one person to another , let alone from one generation to another . we would know much , much less about the world around us . so learning about a topic by asking an expert on that topic , or appealing to authority , helps us gain knowledge , but , it does n't always . even the most highly respected authorities can turn out to be wrong . occasionally this happens because a highly respected authority is dishonest and claims to know something that she or he really does n't know . sometimes it happens just because they make a mistake . they think they know when they do n't know . for example , a number of respected economists did not expect the financial collapse of 2008 . they turned out to be wrong . maybe they were wrong because they were overlooking some important evidence . maybe they were wrong because they were misinterpreting some of the evidence they had noticed . or maybe they were wrong simply because they were reasoning carelessly from the total body of their evidence . but whatever the reason , they turned out to be wrong and many people who trusted their authority ended up losing lots of money , losing lots of other people 's money , on account of that misplaced trust . so while appealing to authority can sometimes provide us with valuable knowledge , it also can sometimes be the cause of monumental errors . it 's important to all of us to be able to distinguish those occasions on which we can safely and reasonably trust authority from those occasions on which we ca n't . but how do we do that ? in order to do that , nothing is more useful than an authority 's track record on a particular topic . if someone turns out to perform well in a given situation much of the time , then it 's likely that he or she will continue to perform well in that same situation , at least in the near term . and this generalization holds true of the testimony of authorities as much as of anything else . if someone can consistently pick winners in both politics and baseball , then we should probably trust him or her to keep on picking winners in both politics or baseball , though maybe not in other things where his or her track record may be less stellar . if other forecasters have a poorer track record on those same two topics , then we should n't trust them as much . so whenever you 're considering whether to trust the testimony of some authority , the first question to ask yourself is , `` what 's their track record on this topic ? '' and notice that you can apply the very same lesson to yourself . your instincts tell you that you 've just met mr . right , but what sort of track record do your instincts have on topics like this one ? have your instincts proven themselves to be worthy of your trust ? just as we judge other people 's testimony by their track record , so , too , we can judge our own instincts by their track record . and this brings us one step closer to an objective view of ourselves and our relation to the world around us .

if someone can consistently pick winners in both politics and baseball , then we should probably trust him or her to keep on picking winners in both politics or baseball , though maybe not in other things where his or her track record may be less stellar . if other forecasters have a poorer track record on those same two topics , then we should n't trust them as much . so whenever you 're considering whether to trust the testimony of some authority , the first question to ask yourself is , `` what 's their track record on this topic ? '' and notice that you can apply the very same lesson to yourself .

you are deciding whether to stay in your hometown and take a low-paying job or whether to move yourself and your family to a new town to take a higher-paying job . you ’ ve never been to the new town before , so you want to find out more information about what it would be like to live there . where will you find that information ? why will you trust those sources ? which sources would you not trust ?

so here we have a sample of vanadium . so vanadium is used to do catalysis , it is used for a whole range of different things . so vanadium is the third of the transition metals and , because it has more electrons , it has really quite interesting chemistry . it is also used in nature and there are a number of mushrooms that are vanadium rich . the amavadins especially is one of the materials . so here if we open , we can see on our reel is a very nice sample of vanadium wire . so this is the elemental vanadium and as you can see it is about 0.1 mm in diameter and there is 20 metres of it on this reel . some of its compounds are very reactive . i can remember that my lab mate paul was working with a compound called vanadium hexacarbonyl . he got a sample it was quite expensive , and he poured it , it came in a sealed glass vessel so he cut open the glass and poured it out and before his eyes it burst into flames and disappeared .

so this is the elemental vanadium and as you can see it is about 0.1 mm in diameter and there is 20 metres of it on this reel . some of its compounds are very reactive . i can remember that my lab mate paul was working with a compound called vanadium hexacarbonyl . he got a sample it was quite expensive , and he poured it , it came in a sealed glass vessel so he cut open the glass and poured it out and before his eyes it burst into flames and disappeared .

what is the property of vanadium compounds that the professor wanted to illustrate with the example of vanadium hexacarbonyl ?

testing , testing , one , two , three . when your band is trying to perform , feedback is an annoying obstacle , but in the grand orchestra of nature , feedback is not only beneficial , it 's what makes everything work . what exactly is feedback ? the key element , whether in sound , the environment or social science , is a phenomenon called mutual causal interaction , where x affects y , y affects x , and so on , creating an ongoing process called a feedback loop . and the natural world is full of these mechanisms formed by the links between living and nonliving things that build resilience by governing the way populations and food webs respond to events . when plants die , the dead material enriches the soil with humus , a stable mass of organic matter , providing moisture and nutrients for other plants to grow . the more plants grow and die , the more humus is produced , allowing even more plants to grow , and so on . this is an example of positive feedback , an essential force in the buildup of ecosystems . but it 's not called positive feedback because it 's beneficial . rather , it is positive because it amplifies a particular effect or change from previous conditions . these positive , or amplifying , loops can also be harmful , like when removing a forest makes it vulnerable to erosion , which removes organic matter and nutrients from the earth , leaving less plants to anchor the soil , and leading to more erosion . in contrast , negative feedback diminishes or counteracts changes in an ecosystem to maintain a more stable balance . consider predators and their prey . when lynx eat snowshoe hares , they reduce their population , but this drop in the lynx 's food source will soon cause their own population to decline , reducing the predation rate and allowing the hare population to increase again . the ongoing cycle creates an up and down wavelike pattern , maintaining a long-term equilibrium and allowing a food chain to persist over time . feedback processes might seem counterintuitive because many of us are used to more predictable linear scenarios of cause and effect . for instance , it seems simple enough that spraying pesticides would help plants grow by killing pest insects , but it may trigger a host of other unexpected reactions . for example , if spraying pushes down the insect population , its predators will have less food . as their population dips , the reduced predation would allow the insect population to rise , counteracting the effects of our pesticides . note that each feedback is the product of the links in the loop . add one negative link and it will reverse the feedback force entirely , and one weak link will reduce the effect of the entire feedback considerably . lose a link , and the whole loop is broken . but this is only a simple example , since natural communities consist not of separate food chains , but networks of interactions . feedback loops will often be indirect , occurring through longer chains . a food web containing twenty populations can generate thousands of loops of up to twenty links in length . but instead of forming a disordered cacophany , feedback loops in ecological systems play together , creating regular patterns just like multiple instruments , coming together to create a complex but harmonious piece of music . wide-ranging negative feedbacks keep the positive feedbacks in check , like drums maintaining a rhythm . you can look at the way a particular ecosystem functions within its unique habitat as representing its trademark sound . ocean environments dominated by predator-prey interactions , and strong negative and positive loops stabilized by self-damping feedback , are powerful and loud , with many oscillations . desert ecosystems , where the turn over of biomass is slow , and the weak feedbacks loops through dead matter are more like a constant drone . and the tropical rainforest , with its great diversity of species , high nutrient turnover , and strong feedbacks among both living and dead matter , is like a lush panoply of sounds . despite their stabilizing effects , many of these habitats and their ecosystems develop and change over time , as do the harmonies they create . deforestation may turn lush tropics into a barren patch , like a successful ensemble breaking up after losing its star performers . but an abandoned patch of farmland may also become a forest over time , like a garage band growing into a magnificent orchestra .

these positive , or amplifying , loops can also be harmful , like when removing a forest makes it vulnerable to erosion , which removes organic matter and nutrients from the earth , leaving less plants to anchor the soil , and leading to more erosion . in contrast , negative feedback diminishes or counteracts changes in an ecosystem to maintain a more stable balance . consider predators and their prey .

negative feedback is called negative because ________ .

a close encounter with a man-eating giant , a sorceress who turns men into pigs , a long-lost king taking back his throne . on their own , any of these make great stories , but each is just one episode in the `` odyssey , '' a 12,000-line poem spanning years of ancient greek history , myth , and legend . how do we make sense of such a massive text that comes from and tells of a world so far away ? the fact that we can read the `` odyssey '' at all is pretty incredible , as it was composed before the greek alphabet appeared in the 8th century bce . it was made for listeners rather than readers , and was performed by oral poets called rhapsodes . tradition identifies its author as a blind man named homer . but no one definitively knows whether he was real or legendary . the earliest mentions of him occur centuries after his era . and the poems attributed to him seem to have been changed and rearranged many times by multiple authors before finally being written down in their current form . in fact , the word rhapsode means stitching together , as these poets combined existing stories , jokes , myths , and songs into a single narrative . to recite these massive epics live , rhapsodes employed a steady meter , along with mnemonic devices , like repetition of memorized passages or set pieces . these included descriptions of scenery and lists of characters , and helped the rhapsode keep their place in the narrative , just as the chorus or bridge of a song helps us to remember the next verses . because most of the tales were familiar to the audience , it was common to hear the sections of the poem out of order . at some point , the order became set in stone and the story was locked into place as the one we read today . but since the world has changed a bit in the last several thousand years , it helps to have some background before jumping in . the `` odyssey '' itself is a sequel to homer 's other famous epic , the `` iliad , '' which tells the story of the trojan war . if there 's one major theme uniting both poems , it 's this : do not , under any circumstances , incur the wrath of the gods . the greek pantheon is a dangerous mix of divine power and human insecurity , prone to jealousy and grudges of epic proportions . and many of the problems faced by humans in the poems are caused by their hubris , or excessive pride in believing themselves superior to the gods . the desire to please the gods was so great that the ancient greeks traditionally welcomed all strangers into their homes with generosity for fear that the strangers might be gods in disguise . this ancient code of hospitality was called xenia . it involved hosts providing their guests with safety , food , and comfort , and the guests returning the favor with courtesy and gifts if they had them . xenia has a significant role in the `` odyssey , '' where odysseus in his wanderings is the perpetual guest , while in his absence , his clever wife penelope plays non-stop host . the `` odyssey '' recounts all of odysseus 's years of travel , but the narrative begins in medias res in the middle of things . ten years after the trojan war , we find our hero trapped on an island , still far from his native ithaca and the family he has n't seen for 20 years . because he 's angered the sea god poseidon by blinding his son , a cyclops , odysseus 's passage home has been fraught with mishap after mishap . with trouble brewing at home and gods discussing his fate , odysseus begins the account of those missing years to his hosts . one of the most fascinating things about the `` odyssey '' is the gap between how little we know about its time period and the wealth of detail the text itself contains . historians , linguists , and archeologists have spent centuries searching for the ruins of troy and identifying which islands odysseus visited . just like its hero , the 24-book epic has made its own long journey through centuries of myth and history to tell us its incredible story today .

how do we make sense of such a massive text that comes from and tells of a world so far away ? the fact that we can read the `` odyssey '' at all is pretty incredible , as it was composed before the greek alphabet appeared in the 8th century bce . it was made for listeners rather than readers , and was performed by oral poets called rhapsodes .

when did the greek alphabet first appear ?

you 're standing at the ready inside the goal when suddenly , you feel an intense itch on the back of your head . we 've all experienced the annoyance of an inconvenient itch , but have you ever pondered why we itch in the first place ? the average person experiences dozens of individual itches each day . they can be triggered by all sorts of things , including allergic reactions , dryness , and even some diseases . and then there are the mysterious ones that pop up for no reason at all , or just from talking about itching . you 're scratching your head right now , are n't you ? anyhow , let 's take one of the most common sources : bug bites . when a mosquito bites you , it releases a compound into your body called an anticoagulant that prevents your blood from clotting . that compound , which we 're mildly allergic to , triggers the release of histamine , a chemical that makes our capillaries swell . this enables increased blood flow , which helpfully accelerates the body 's immune response to this perceived threat . that explains the swelling , and it 's the same reason pollen can make your eyes puff up . histamine also activates the nerves involved in itching , which is why bug bites make you scratch . but the itchy sensation itself is n't yet fully understood . in fact , much of what we do know comes from studying the mechanics of itching in mice . researchers have discovered that itch signals in their skin are transmitted via a subclass of the nerves that are associated with pain . these dedicated nerves produce a molecule called natriuretic polypetide b , which triggers a signal that 's carried up the spinal cord to the brain , where it creates the feeling of an itch . when we scratch , the action of our fingernails on the skin causes a low level pain signal that overrides the itching sensation . it 's almost like a distraction , which creates the sensation of relief . but is there actually an evolutionary purpose to the itch , or is it simply there to annoy us ? the leading theory is that our skin has evolved to be acutely aware of touch so that we 're equipped to deal with risks from the outside world . think about it . our automatic scratching response would dislodge anything harmful that 's potentially lurking on our skin , like a harmful sting , a biting insect , or the tendrils of a poisonous plant . this might explain why we do n't feel itching inside our bodies , like in our intestines , which is safe from these external threats , though imagine how maddening that would be . in some people , glitches in the pathways responsible for all of this can cause excessive itching that can actually harm their health . one extreme example is a psychological condition called delusory parasitosis where people believe their bodies are infested with mites or fleas scurrying over and under their skin , making them itch incessantly . another phenomenon called phantom itching can occur in patients who 've had amputations . because this injury has so severely damaged the nervous system , it confuses the body 's normal nerve signaling and creates sensations in limbs that are no longer there . doctors are now finding ways to treat these itching anomalies . in amputees , mirrors are used to reflect the remaining limb , which the patient scratches . that creates an illusion that tricks the brain into thinking the imaginary itch has been satisfied . oddly enough , that actually works . researchers are also searching for the genes involved in itching and developing treatments to try and block the pathway of an itch in extreme cases . if having an unscratchable itch feels like your own personal hell , dante agreed . the italian poet wrote about a section of hell where people were punished by being left in pits to itch for all eternity .

because this injury has so severely damaged the nervous system , it confuses the body 's normal nerve signaling and creates sensations in limbs that are no longer there . doctors are now finding ways to treat these itching anomalies . in amputees , mirrors are used to reflect the remaining limb , which the patient scratches .

how do you think researchers could go about finding a treatment—or even a cure—for chronic itching ?

do you have a friend or a sibling that 's always competing with you to see who 's the fastest ? our alien friends bleebop and mark are having the same debate with their custom-built rockets , and they 've asked us to be the judge of a space race to their moon . the only problem is that they are starting from different asteroids . bleebop is on an asteroid 240 miles from the moon , and mark is on one 150 miles away . do n't worry , it 's not rocket science . solving this equation is as simple as dirt . we can decide who the winner is using the d=rt formula , or dirt . this stands for distance equals rate times time . in the case of bleebop and mark , we will only know the distance they traveled and the time it took for them to get to the finish line . it 'll be up to us to find the rate and who is faster . let 's turn to the race now and see what information we get . three , two , one , blast off ! bleebop and mark 's rockets go zipping across the galaxy towards their moon , dodging clunky meteorites and loopy space buggies . after a few close calls with a wandering satellite , mark arrives first in two hours , and bleebop gets there one hour later . looks like mark has the faster rocket , but let 's check out the results with our dirt equation . begin by setting up a chart . make four columns and three rows . use dirt to remember what to fill in . each rocket will have information for distance , rate , and time . mark 's rocket went 150 miles , we do n't know the rate , and he got there in 2 hours . bleebop 's rocket went 240 miles , we do n't know the rate , and the time is 1 hour after mark , or 3 hours . because we do n't know mark or bleebop 's rate , that number is going to be a variable in each equation , which we 'll represent with x . we 'll solve the equation for the variable to find its value . mark finished first , so start with his rocket . remembering dirt , write down d=rt . 150 miles equals x times 2 hours . divide both sides by 2 hours . this will leave x isolated on the right side of the equation . 150 miles divided by 2 hours is 75 miles over 1 hour . mark 's rate is 75 miles per hour . that 's what mph means . it 's the amount of miles over one hour . still think mark is faster ? let 's set up the same equation for bleebop and see . d=rt 240 miles equals x times 3 hours . divide both sides by 3 hours . this will leave x isolated on the right side of the equation . 240 miles divided by 3 hours is 80 miles over 1 hour . bleebop 's rate is 80 miles per hour . wow , even though bleebop got there one hour later , it turns out he had the faster rocket . mark seems pretty upset , but with aliens , you can never really tell . thanks to dirt , you now know how to calculate distance , rate , and time . in what other situations can you use the distance formula ? you do n't even need to be watching a space race . as long as you know two pieces of information for the formula d=rt , you can calculate any moving vehicle or object . now , the next time you 're in a car , you can let your friends know exactly when you 'll be arriving , how fast you 're going , or the distance you 'll travel . it 's as simple as dirt .

this will leave x isolated on the right side of the equation . 240 miles divided by 3 hours is 80 miles over 1 hour . bleebop 's rate is 80 miles per hour .

how many miles would you travel if you drove a car 65 mph for 3 hours ?

translator : tom carter reviewer : bedirhan cinar of all the spectacles mankind has viewed through a telescope , there are few lovelier than a spiral galaxy . majestic whirlpools of stars , they rotate in a stately and predictable dance . the fact that we see many billions of them in our telescopes tells us they are both common and stable . it is perhaps surprising that it is relatively easy to understand the inner workings of these cosmic pinwheels . by combining physical principles worked out by sir isaac newton in the late 17th century , with the observed amount of mass in a galaxy , scientists can calculate the rates at which these galaxies rotate . using these techniques , astronomers predict how fast stars at different distances from the center of the galaxy should move . stars very close to the center move slowly . that 's because there is very little mass between them and the center of the galaxy to pull them along . stars a bit further away move faster , because they are being pulled by all of the stars in between them and the center . as we get really far away , the stars are predicted to move slowly again . their great distance reduces gravity to a gentle tug , so they move leisurely in their orbits . knowing this , scientists looked at the galaxies and measured how fast stars were moving . to their surprise , they found that while the stars closer to the center of the galaxy behaved as predicted , those further away moved far too quickly . this observation was devastating to the tradtional theories of gravity and motion . if the stars were moving as fast as their measurement suggested , galaxies should have torn themselves apart . it was a crisis , and astronomers and physicists scrambled to find a mistake in their calculation . was newton 's theory of gravity wrong ? was his theory of motion wrong ? or was it possible that astronomers had incorrectly measured the galaxy 's mass ? all options were investigated , and all were ruled out . except one . today , scientists believe that the answer lies in a previously unknown kind of matter , called dark matter . this dark matter can be envisioned as a cloud which surrounds most galaxies . this matter is very unusual . it is affected by gravity , but it is invisible to visible light and all other forms of electromagnetic radiation . the name `` dark matter '' originates in this form of matter 's inability to emit or absorb light . dark matter adds to the gravity of the galaxy and explains the orbital speed of stars far from the galactic center . dark matter has not yet been directly observed , but scientists believe that it is likely to be real , mostly because the other options have been ruled out . using dozens of approaches , astronomers and physicists continue to search for direct evidence that would prove that the dark matter hypothesis is true . this question is one of the most important physics research questions of the 21st century .

dark matter has not yet been directly observed , but scientists believe that it is likely to be real , mostly because the other options have been ruled out . using dozens of approaches , astronomers and physicists continue to search for direct evidence that would prove that the dark matter hypothesis is true . this question is one of the most important physics research questions of the 21st century .

every day physicists learn more about our universe by probing deeper and deeper into space . if you were a physicists , what questions would you want to try to answer ?

imagine a group of people . how big do you think the group would have to be before there 's more than a 50 % chance that two people in the group have the same birthday ? assume for the sake of argument that there are no twins , that every birthday is equally likely , and ignore leap years . take a moment to think about it . the answer may seem surprisingly low . in a group of 23 people , there 's a 50.73 % chance that two people will share the same birthday . but with 365 days in a year , how 's it possible that you need such a small group to get even odds of a shared birthday ? why is our intuition so wrong ? to figure out the answer , let 's look at one way a mathematician might calculate the odds of a birthday match . we can use a field of mathematics known as combinatorics , which deals with the likelihoods of different combinations . the first step is to flip the problem . trying to calculate the odds of a match directly is challenging because there are many ways you could get a birthday match in a group . instead , it 's easier to calculate the odds that everyone 's birthday is different . how does that help ? either there 's a birthday match in the group , or there is n't , so the odds of a match and the odds of no match must add up to 100 % . that means we can find the probability of a match by subtracting the probability of no match from 100 . to calculate the odds of no match , start small . calculate the odds that just one pair of people have different birthdays . one day of the year will be person a 's birthday , which leaves only 364 possible birthdays for person b . the probability of different birthdays for a and b , or any pair of people , is 364 out of 365 , about 0.997 , or 99.7 % , pretty high . bring in person c. the probability that she has a unique birthday in this small group is 363 out of 365 because there are two birthdates already accounted for by a and b . d 's odds will be 362 out of 365 , and so on , all the way down to w 's odds of 343 out of 365 . multiply all of those terms together , and you 'll get the probability that no one shares a birthday . this works out to 0.4927 , so there 's a 49.27 % chance that no one in the group of 23 people shares a birthday . when we subtract that from 100 , we get a 50.73 % chance of at least one birthday match , better than even odds . the key to such a high probability of a match in a relatively small group is the surprisingly large number of possible pairs . as a group grows , the number of possible combinations gets bigger much faster . a group of five people has ten possible pairs . each of the five people can be paired with any of the other four . half of those combinations are redundant because pairing person a with person b is the same as pairing b with a , so we divide by two . by the same reasoning , a group of ten people has 45 pairs , and a group of 23 has 253 . the number of pairs grows quadratically , meaning it 's proportional to the square of the number of people in the group . unfortunately , our brains are notoriously bad at intuitively grasping non-linear functions . so it seems improbable at first that 23 people could produce 253 possible pairs . once our brains accept that , the birthday problem makes more sense . every one of those 253 pairs is a chance for a birthday match . for the same reason , in a group of 70 people , there are 2,415 possible pairs , and the probability that two people have the same birthday is more than 99.9 % . the birthday problem is just one example where math can show that things that seem impossible , like the same person winning the lottery twice , actually are n't unlikely at all . sometimes coincidences are n't as coincidental as they seem .

every one of those 253 pairs is a chance for a birthday match . for the same reason , in a group of 70 people , there are 2,415 possible pairs , and the probability that two people have the same birthday is more than 99.9 % . the birthday problem is just one example where math can show that things that seem impossible , like the same person winning the lottery twice , actually are n't unlikely at all .

how many people would you need in a group to ensure a 99.9 % probability of two of them sharing the same birthday ?

we have some good reasons to completely switch over to solar power . it 's cheaper in many cases , and definitely more sustainable than our dependance on traditional power plants that use resources like coal , which will eventually run out . so why do n't we replace these traditional plants with solar energy ? because there 's one factor that makes solar power very unpredictable : cloud cover . as the sun 's rays move towards earth , some get absorbed by the earth 's atmosphere , some are reflected back into outer space , but the rest make it to the earth 's surface . the ones that are n't deviated are called direct irradiance . the ones that are deflected by clouds are called diffuse irradiance . and those rays that first get reflected by a surface , like a nearby building , before reaching the solar energy system are called reflected irradiance . but before we can examine how clouds affect the sun 's rays and electricity production , let 's see how these solar energy systems work . first up , we have solar towers . these are made up of a central tower surrounded by a huge field of mirrors that track the sun 's path and focus only the direct rays onto a single point on the tower , kind of like an eager beachgoer . the heat generated by these rays is so immense that it can be used to boil water producing steam that drives a traditional turbine , which makes electricity . but when we say solar energy systems , we 're usually talking about photovoltaics , or solar panels , which are the systems most commonly used to generate solar power . in solar panels , photons from the sun 's rays hit the surface of a panel , and electrons are released to get an electric current going . solar panels can use all types of irradiance , while solar towers can only use direct irradiance , and this is where clouds become important because depending on their type and location relative to the sun , they can either increase or decrease the amount of electricity produced . for instance , even a few cumulus clouds in front of the sun can reduce the electricity production in solar towers to almost zero because of this dependence on direct rays . in solar panels , those clouds would decrease energy output as well , though not as much because solar panels can use all types of irradiance . however , all this depends on the clouds exact positioning . due to reflection , or a particular phenomeon called mie scattering , the sun 's rays can actually be focused forward by clouds to create a more than 50 % increase in the solar irradiance reaching a solar panel . if this potential increase is n't accounted for , it could damage the solar panel . why does this matter ? well , you would n't want this lesson to stop just because a cloud passed over the panel on your roof . in solar towers , huge tanks of molten salt or oil can be used to store any excess heat and use it when needed , so that 's how they manage the problem of fluctuating solar irradiance to smooth out electricity production . but in the case of solar panels , there currently is n't any way to affordably store extra energy . that 's where traditional power plants come in because to correct for any fluctuations in these solar powered plants , extra electricity from traditional sources always needs to be available . but then why are n't these tradtional power plants just used as a backup , instead of us humans depending on them as our main sources of energy ? because it 's impossible for an employee at a coal fired or a nuclear plant to turn a knob to produce more or less electricity depending on how many clouds there are in the sky . the response time would simply be too slow . instead , to accommodate these fluctuations , some extra electricity from traditional power plants is always being produced . on clear sky days , that extra electricity might be wasted , but when cloudy skies prevail , it 's what fills the gap . this is what we currently depend on for a constant supply of energy . for this reason , a lot of researchers are interested in forcasting the motion and formation of clouds through satellite images or cameras that look up at the sky to maximize the energy from solar power plants and minimize energy waste . if we could accomplish that , you 'd be able to enjoy this video powered solely by the sun 's rays , no matter what the weather , although if the sun is shining , you may be tempted to venture outside to go and do a different kind of cloud gazing .

for instance , even a few cumulus clouds in front of the sun can reduce the electricity production in solar towers to almost zero because of this dependence on direct rays . in solar panels , those clouds would decrease energy output as well , though not as much because solar panels can use all types of irradiance . however , all this depends on the clouds exact positioning . due to reflection , or a particular phenomeon called mie scattering , the sun 's rays can actually be focused forward by clouds to create a more than 50 % increase in the solar irradiance reaching a solar panel .

how many main types of clouds are there ?

for most jobs , it 's understood that you can be fired , whether for crime , incompetence , or just poor performance . but what if your job happens to be the most powerful position in the country , or the world ? that 's where impeachment comes in . impeachment is n't the same as actually removing someone from office . like an indictment in criminal court , it 's only the formal accusation that launches a trial , which could end in conviction or acquittal . originating in the united kingdom , impeachment allowed parliament to vote for removing a government official from office even without the king 's consent . although this was an important check on royal power , the king could n't be impeached because the monarch was considered the source of all government power . but for the founders of the american republic , there was no higher authority beyond the people themselves . and so impeachment was adopted in the united states as a power of congress applying to any civil officers , up to and including the president . although demands for impeachment can come from any members of the public , only the house of representatives has the power to actually initiate the process . it begins by referring the matter to a committee , usually the house committee on rules and the house committee on the judiciary . these committees review the accusations , examine the evidence , and issue a recommendation . if they find sufficient grounds to proceed , the house holds a separate vote on each of the specific charges , known as articles of impeachment . if one or more passes by a simple majority , the official is impeached and the stage is set for trial . the actual trial that follows impeachment is held in the senate . selected members of the house , known as managers , act as the prosecution , while the impeached official and their lawyers present their defense . the senate acts as both judge and jury , conducting the trial and deliberating after hearing all the arguments . if it 's the president or vice president being impeached , the chief justice of the supreme court presides . a conviction requires a supermajority of two-thirds and results in automatic removal from power . depending on the original charges , it can also disqualify them from holding office in the future and open them to standard criminal prosecution . so what exactly can get someone impeached ? that 's a bit more complicated . unlike in the united kingdom , impeachment in the u.s. pits an elected legislature against other democratically elected members of government . therefore , to prevent the process from being used as a political weapon , the constitution specifies that an official can only be impeached for treason , bribery , or other high crimes and misdemeanors . that still leaves a lot of room for interpretation , not to mention politics , and many impeachment trials have split along partisan lines . but the process is generally understood to be reserved for serious abuses of power . the first official to be impeached was tennesse senator william blount in 1797 for conspiring with britain to cease the spanish colony of louisiana . since then , the house has launched impeachment investigations about 60 times , but only 19 have led to actual impeachment proceedings . the eight cases that ended in a conviction and removal from office were all federal judges . and impeachment of a sitting president is even more rare . andrew johnson was impeached in 1868 for attempting to replace secretary of war edwin stanton without consulting the senate . over a century later , bill clinton was impeached for making false statements under oath during a sexual harassment trial . both were ultimately acquitted when the senate 's votes to convict fell short of the required two-thirds majority . and contrary to popular belief , richard nixon was never actually impeached for the watergate scandal . he resigned before it could happen knowing he would almost certainly be convicted . theoretically , the u.s. government is already designed to prevent abuses of power , limiting different branches through a system of checks and balances , term limits , and free elections . but impeachment can be seen as an emergency brake for when these safeguards fail .

depending on the original charges , it can also disqualify them from holding office in the future and open them to standard criminal prosecution . so what exactly can get someone impeached ? that 's a bit more complicated .

which of the following presidents was not formally impeached ?

nicolas steno is rarely heard of outside intro to geology , but anyone hoping to understand life on earth should see how steno expanded and connected those very concepts : earth , life , and understanding . born niels stensen in 1638 denmark , son of a goldsmith , he was a sickly kid whose school chums died of plague . he survived to cut up corpses as an anatomist , studying organs shared across species . he found a duct in animal skulls that sends saliva to the mouth . he refuted descartes ' idea that only humans had a pineal gland , proving it was n't the seat of the soul , arguably , the debut of neuroscience . most remarkable for the time was his method . steno never let ancient texts , aristotelian metaphysics , or cartesian deductions overrule empirical , experimental evidence . his vision , uncluttered by speculation or rationalization , went deep . steno had seen how gallstones form in wet organs by accretion . they obeyed molding principles he knew from the goldsmith trade , rules useful across disciplines for understanding solids by their structural relationships . later , the grand duke of tuscany had him dissect a shark . its teeth resembled tongue stones , odd rocks seen inside other rocks in malta and the mountains near florence . pliny the elder , old roman naturalist , said these fell from the sky . in the dark ages , folks said they were snake tongues , petrified by saint paul . steno saw that tongue stones were shark teeth and vice versa , with the same signs of structural growth . figuring similar things are made in similar ways , he argued the ancient teeth came from ancient sharks in waters that formed rock around the teeth and became mountains . rock layers were once layers of watery sediment , which would lay out horizontally , one atop another , oldest up to newest . if layers were deformed , tilted , cut by a fault or a canyon , that change came after the layer formed . sounds simple today ; back then , revolutionary . he 'd invented stratigraphy and laid geology 's ground work . by finding one origin for shark teeth from two eras by stating natural laws ruling the present also ruled the past , steno planted seeds for uniformitarianism , the idea that the past was shaped by processes observable today . in the 18th and 19th centuries , english uniformitarian geologists , james hutton and charles lyell , studied current , very slow rates of erosion and sedimentation and realized the earth had to be way older than the biblical guestimate , 6000 years . out of their work came the rock cycle , which combined with plate tectonics in the mid-twentieth century to give us the great molten-crusting , quaking , all-encircling theory of the earth , from a gallstone to a 4.5 billion-year-old planet . now think bigger , take it to biology . say you see shark teeth in one layer and a fossil of an organism you 've never seen under that . the deeper fossil 's older , yes ? you now have evidence of the origin and extinction of species over time . get uniformitarian . maybe a process still active today caused changes not just in rocks but in life . it might also explain similarities and differences between species found by anatomists like steno . it 's a lot to ponder , but charles darwin had the time on a long trip to the galapagos , reading a copy of his friend charles lyell 's `` principles of geology , '' which steno sort of founded . sometimes giants stand on the shoulders of curious little people . nicolas steno helped evolve evolution , broke ground for geology , and showed how unbiased , empirical observation can cut across intellectual borders to deepen our perspective . his finest accomplishment , though , may be his maxim , casting the search for truth beyond our senses and our current understanding as the pursuit of the beauty of the as yet unknown . beautiful is what we see , more beautiful is what we know , most beautiful , by far , is what we do n't .

nicolas steno is rarely heard of outside intro to geology , but anyone hoping to understand life on earth should see how steno expanded and connected those very concepts : earth , life , and understanding . born niels stensen in 1638 denmark , son of a goldsmith , he was a sickly kid whose school chums died of plague .

book 1 presents proof that book 2 is wrong about something ! what do you do ?

translator : andrea mcdonough reviewer : jessica ruby worst case scenario : zombie apocalypse . how will you survive ? you might be surprised to find out how much geography skills can help you fend off doom . by geography , i mean analyzing the world around you . one geographic concept that could really help you out in a zombie apocalypse is movement . so , first , what moves ? people move , animals move , and , while sometimes slowly , zombies move as well . but that 's not all . goods move , too . goods can be resources , such as food supplies and weapons . people or zombies tend to move these . so , if you see a big pile of zombie supplies where there was n't one before , you 're probably on the trail . ideas also move . ideas can include entertainment , zombie movies , news and information about zombie attacks , and architecture , or how to build a safe shelter . and , second , why do people or zombies move ? when people , animals , or zombies move , it 's called migration . two concepts that affect migration are push and pull factors . push factors will make you want to leave somewhere . pull factors make you want to go to a place . a lack of resources , unstable economy , or high crime rate might be push factors making people want to move . nice weather , a good economy , or lots of resources would be pull factors for lots of people , enticing them to move . while zombies are definitely a push factor for humans , a city full of people would be a pull factor for hungry zombies who want to eat humans . there are some things that make movement easier for people or zombies . waterways and highways can make traveling faster . moving across clear , open space is easier than a tough terrain . and just as land forms can create boundaries that affect movement , so can political boundaries , like a border gate , for example . so , how can you analyze these movement factors to help your chance of survival ? there are three basic steps . one - identify the points or locations to analyze . what are your options ? two - find what connects them . are there highways , waterways , or open land ? and three - find the patterns of movement that happen over that connection . do people or goods move across it ? by comparing relationships between different places , you can see what connections they have . for example , pick two cities . look at the highway connecting them . if people use that highway to commute to work , those cities have a strong relationship . but this other city over here does n't have a direct connection to the other cities . there 's even a river in the way . it does n't have as strong of a relationship . if a zombie outbreak started here , which city would you rather start out in ? where would you flee to ? so , how do you decide where to go in a zombie apocalypse ? do you just run in a random direction ? or do you use your geographic skills to lead your camp of survivors to safety ? if you want to stay alive , it helps to understand how and why we move .

are there highways , waterways , or open land ? and three - find the patterns of movement that happen over that connection . do people or goods move across it ?

which of the following patterns would create the strongest connection between two locations ?

mysteries of vernacular : x-ray , a form of electromagnetic radiation capable of penetrating solids . the word x-ray harkens back to the work of rene descartes , a french philosopher , mathematician , and writer in the 17th century . one of descartes innumerable contributions to the world of numbers was the invention of a simple yet brilliant convention most people take for granted today : the representation of unknowns within an equation as x , y , and z . when the german scientist wilhelm rontgen discovered what we now call x-rays in the late 19th century , he gave them the name x-strahlen . strahlen is german for shine , and x , of course , represented the unknown nature of the radiation rontgen had discovered , the x-factor , so to speak . the english translation maintained the x but replaced the german shine with ray , meaning a beam of light . coincidentally , in mathematics , the word ray refers to a line with a point of origin that has no end and extends to infinity , bringing us neatly back to the unknown . today we understand what x-radiation is , and in spite of the humble objections of its discoverer , it is also commonly called rontgen radiation , eliminating with the x the fundamental mystery of its nature .

the english translation maintained the x but replaced the german shine with ray , meaning a beam of light . coincidentally , in mathematics , the word ray refers to a line with a point of origin that has no end and extends to infinity , bringing us neatly back to the unknown . today we understand what x-radiation is , and in spite of the humble objections of its discoverer , it is also commonly called rontgen radiation , eliminating with the x the fundamental mystery of its nature .

in mathematics , the word ray refers to a line with a point of origin that has _______ .

would you believe that walruses , rattlesnakes , and parakeets all once lived in the same house ? let 's go back about 350 million years . look around . steamy swamps and rain forests of horsetails and ferns cover the region . amphibians are the dominant land vertebrates . they range in size from newts to crocodiles . and all require water to do their egg laying . if they do n't go to the water , their shell-less , jelly-like eggs will dry out . because of this hazard , they spend most of their time living in or near fresh water . that is , until a breakthrough in evolution changes everything : the amniotic egg . the amniotic egg is shelled , waterproof , and can be laid on dry land . it is produced by the amniotes , a new group of animals named after their revolutionary egg . the first amniote is a tetrapod , a four-legged animal , resembling a small lizard . while some amphibians can walk around on land and bury their eggs in wet soil or highly humid areas , nothing before the amniotes has the ability to lay its eggs on completely dry land . because of this evolved egg , the amniotes are the first animals with the ability to live a fully terrestrial life . but , despite their move inland , the amniotes have not abandoned their pond-dwelling upbringing . in fact , the amniotic egg brings the pond with them by enclosing the aquatic environment within its shell . this is achieved by four main upgrades that are unique to amniotic eggs . let 's take a closer look . the first development is the most obvious : the egg 's protective shell . it 's tough but flexible , and has a leathery surface , still seen in reptile eggs today . the shell protects the eggs from predators , bacteria , damage , and drying out . but , unlike the walls of a fish tank , the shell of the amniotic egg is porous , allowing oxygen to pass through so that the growing amniote inside does n't suffocate . the next two developments are two separate membranes that work together like a pair of lungs . they bring oxygen into the embryo while removing carbon dioxide . the first is the chorion , which is the protective layer that oxygen passes through after entering the shell 's tiny pores . you may recognize the chorion as the thin skin you peel away on a hard boiled egg . think of this waterproof membrane as the in and out doors of the egg . it 's the entrance for oxygen and exit for carbon dioxide . the membrane working with the chorion is the allantois . if the chorion is the doors , then the allantois is essentially the lobby of the building . it directs the oxygen and carbon dioxide while simultaneously storing unneeded waste from the embryo . the chorion and the allantois make sure the embryo has everything it needs and gets rid of anything it does n't . the last and perhaps the most important development is the amnion , the membrane for which the egg is named . the amnion is also contained within the chroion and holds the fluid in which the embryo floats . because it has left the watery world of the amphibians , the amnion is necessary for preventing the embryo from drying out . it is the transportable pond that allows the amniote to lay the egg on dry land . its fluid also protects the embryo from any collisions or rough landings , like a shock absorber on your bike or car . together , the shell and these four membranes create a safe , watery environment for the embryo to grow and develop . the new amniote offspring will continue the process of vertebrate evolution as it explores new land away from the water . they will spend the next million years splitting into two distinct groups : the synapsids and sauropsids . synapsida is the group of animals that contain mammals , while sauropsida is the group that contains reptiles , birds , and dinosaurs . these two amniotic groups collectively contain the walruses , rattlesnakes , and parakeets we know today . like a family reunion , with relatives of every shape and size , coming together from different corners of the earth , these animals can all call one place home : the amniotic egg .

because of this hazard , they spend most of their time living in or near fresh water . that is , until a breakthrough in evolution changes everything : the amniotic egg . the amniotic egg is shelled , waterproof , and can be laid on dry land .

the amniotic egg evolved __________ years ago .

so today we ’ re going to look at bromine . so this is bromine , it ’ s a small sample , about 2 or 3 grammes . bromine is a red liquid . there are not many elements that are actually liquid : caesium , mercury , gallium and bromine . it ’ s this beautiful red colour and it has an unpleasant smell . bromine comes from the greek word bromos which means stench , a really horrible smell . in fact this bromine probably comes from near where i did my degree because in europe , the second , well the largest producer of bromine in europe is the octale company which is based on amlwch on anglesey which is where i went last weekend . so it ’ s a nice sample . we ’ re going to get it out of this bottle now because it ’ s sealed in because it ’ s obviously very , very volatile as a liquid and we ’ re going to cut the glass , get it out and look at its chemistry . thanks neil . bromine is also quite a common element in the world . it is found in sea water as bromide , it ’ s also found particularly in the dead sea between israel and jordan where there are very concentrated solutions of bromide . so you can make bromine just by bubbling chlorine into a solution of bromide like dead sea water and the chlorine displaces the bromine and forms chloride and the bromine just comes out as red fumes which you can catch . so neil ’ s now using a glass knife to open the ampoule and he ’ s going to pour the liquid bromine into this small evaporating dish inside our fume hood . now instantly you can see the vapour of the bromine coming off . you see all those really nice orange fumes which are coming off from the liquid and then they ’ re being drawn away by our fume hood . bromine has a number of uses , it is particularly used in plastics as an additive to try and make them less flammable . if you ’ re watching your videos on a screen which has a plastic surround almost certainly that surround will contain bromine to make it less flammable . bromine itself is strongly oxidising so we thought we ’ d do an oxidation reaction today . and that reaction is to take another element and here we have aluminium , simple aluminium foil , like you wrap your dinner in , and we ’ re now going to put some small amounts of aluminium foil into the bromine liquid . so we put it in and it generally takes a few moments for this reaction to start but as i said , the aluminium is strongly oxidising . it oxidises and generates aluminium tribromide . which is a beautiful nice material but it ’ s a strongly exothermic reaction . so let ’ s see what happens . this may initiate . it is also an unusual element because elements have different isotopes these are the same form of the element but with different numbers of neutrons in the atoms so they weigh different amounts . usually you get one isotope that is very common and another one that is only a small amount . bromine is unusual because it has two isotopes 79 and 81 which have almost equal quantities in nature so that they have approximately 51 % of one and 49 % of the other which is really quite unusual . so here we see the exothermic oxidation and formation of aluminium tribromide . so the reaction is strongly exothermic which means it gives out energy and that energy then evaporates off excess bromine which you can see coming off as a vapour . a wonderful exothermic reaction : very , very fast , very , very rapid forming aluminium tribromide . bromine is quite dangerous but it is less dangerous than chlorine because it ’ s a liquid at room temperature . chlorine is a gas so if you let out chlorine it disperses everywhere . with bromine it is a heavy gas which can disperse rather more slowly . so now as the reaction cools down we can see that the volume of bromine is significantly reduced . reacts with water quite well .

it is found in sea water as bromide , it ’ s also found particularly in the dead sea between israel and jordan where there are very concentrated solutions of bromide . so you can make bromine just by bubbling chlorine into a solution of bromide like dead sea water and the chlorine displaces the bromine and forms chloride and the bromine just comes out as red fumes which you can catch . so neil ’ s now using a glass knife to open the ampoule and he ’ s going to pour the liquid bromine into this small evaporating dish inside our fume hood .

how can you obtain bromine from bromide solution ?

sunscreen comes in many forms , each with its own impacts on your body and the environment . with so many options , how do you choose which sunscreen is best for you ? to answer that question , we first have to understand how sunscreens work . sunlight is composed of electromagnetic waves and is our primary source of ultraviolet radiation , which has a shorter wavelength than visible light and carries more energy . uva , uvb , and uvc are classified according to their wavelengths . short wavelength uvc never reaches the earth 's surface , but uvb and uva do . medium wavelength uvb rays can enter the skin 's superficial layers and long length uva rays can penetrate into the deeper layers . uvb in small amounts actually helps us make vitamin d , which enables our bodies to build and maintain strong bones . however , prolonged exposure to uva and uvb can damage dna , age your skin , and promote the development of potentially deadly skin cancer . sunscreen protects your skin either physically by deflecting uv rays with an inorganic blocker like zinc oxide or titanium dioxide , or chemically by using carbon-based compounds to absorb uv photons that are then harmlessly dissipated as heat . so , what differentiates one sunscreen from another ? when we choose a sunscreen , we can compare application method , the spf , and the active ingredients . sprays can be convenient to put on , especially when you 're wet , but a recent study found that most people do n't apply a thick enough layer to get full protection . and the possible health risks of inhaling sunscreen compounds from a spray cloud might make you consider reaching for that bottle of lotion instead . opt for a sunscreen with an spf of at least 15 , although 30 is better . spf is a nonlinear scale of how much uvb radiation is needed to give protected skin a sunburn . spf 15 does a pretty good job by blocking 93 % of uvb rays . you get a slight increase as spf goes up , with spf 30 blocking 97 % , and 50 blocking 98 % . spf is based on the quantity of solar exposure . so how much time you have before you start to burn really depends on a long list of factors , including your genetics , and when , where , and how you spend your time in the sun . even though us marketed sunscreens have been deemed safe by the fda , scientists are still researching the effects of many active ingredients on the human body . so if you 're worried about potential irritants , look for mineral-based formulas with zinc oxide or titanium dioxide . even though they may go on a bit thick at first , they 're less irritating than carbon-based chemical sunscreens . these mineral-based sunscreens are preferential for the environment , too . if you plan on catching rays while splashing in a river or the ocean , keep in mind that carbon-based chemical sunscreens can harm marine life . take coral reefs , for example . although they cover less than 1 % of the earth 's underwater surface , they 're home to nearly 25 % of all fish species , making them the most diverse and productive marine ecosystems . research shows that carbon-based chemical sunscreen ingredients , like oxybenzone , butylparaben , octinoxate , and 4mbc contribute to a stress condition called coral bleaching in corals , which are living creatures . exposure to these organic compounds results in the death of the coral 's symbiotic algae . in addition to providing a reliable food source , these algae give coral their brilliant rainbow of colors . without them , corals turn a bleached white and are susceptible to disease and possibly death . and once the coral dies , the entire reef ecosystem is not far behind . so you 're now ready to make an informed choice when picking out your next sunscreen . spf is clearly labeled on the front . on the back under `` active ingredients , '' you can find whether zinc oxide , titanium dioxide , and those coral-harming components are present . taking a bit more time to check can be well worth it for both you and the environment .

sunscreen comes in many forms , each with its own impacts on your body and the environment . with so many options , how do you choose which sunscreen is best for you ?

ultraviolet radiation comes in which of these forms ?

on valentine 's day 2013 , a thai couple broke the world record for longest kiss , when they sucked face for over 58 hours . 58 hours ! wo-how ? ! you could drive across the entire united states in less time ! these lovebirds could n't even sit down that whole time , and had to pee while kissing . the logistics of which i do n't want to think about . still , got me thinking , when the first two humans brushed lips , could they ever have imagined it would end with chapped lips and a cash prize , and where did kissing get it 's start anyway ? the earliest reference to kissing is out of a sanskrit text from india , dating back to 1500 bc . now that does n't mean that folks were n't snogging before then , that 's just the first record we have . some believe that alexander the great 's army spread kissing from india to the middle east . from there , it caught on in ancient greece and rome , perhaps more as a display of deference than romance . slaves kissed the hands of their masters as a sign of submission . men of standing kissed each other on the cheek in greeting , and even today it 's important to note that there are lots of different types of , and reasons for , kissing , from ritual to friendship to comfort to passion . to a guinness book title , apparently . by the middle ages , people all over europe were swapping spit , and by the 19th century we actually gave the study of kissing the decidedly un-sexy title of philematology . rrrr . so , why do we kiss ? well , there are two theories about it 's origins , one maintains that it 's instinctual , the other believes that its a learned behavior . folks in the instinctual camp point out that bonobos and some other animals seem to display kissing type behavior as an expression of comfort , expression and bonding , but critics of this theory like to point out that ten percent of human cultures do n't even kiss , or at least have n't until fairly recently . various tribes in africa and some australian aborigines and tahitians are among groups that do n't seem to kiss , they probably just think that we 're super weird . so some believe that kissing is a learned behavior that evolved from regurgitation . oh yeah . hot stuff . no , i do not mean barfing . i mean mothers chewing solid food and feeding it to their babies , mouth to mouth , like birds , wolves and lots of other animals do . this type of feeding not only helped pre-digest food , but also established bonding and intimacy . and it is n't really a stretch to picture it shifting to kisses as a sign of affection . from there , presumably , people just realized that sucking face felt like all good and tingly , which is a compelling reason to do a lot of things . our lips and tongues are jam-packed with sensitive nerve endings that light up our brains pleasure receptors like a coney island rollercoaster . if it 's a really good , passionate kiss , our cheeks flush as blood vessels expand , our pupils dilate , we get all dreamy eyed , and our heart rates increase , pumping more oxygen into the brain . suddenly , we 're riding a wave of feel-good hormones like dopamine , serotonin , oxytocin and adrenaline . that is the chemical cocktail that makes you feel like a family of bees just moved into your belly . interesting , a males testosterone can pass into a females mouth from kissing , making her more receptive to sex , baby-making , and the passing along of genes , which is , of course , evolution 's ultimate goal . and smooching does n't just feel good to most people , it 's also good for you , in several different ways . it burns calories of course , reduces stress and it might just help you select the right kind of mate . gettin ' all up in someone 's grill gives you great access to a nose full of pheromones , those subtle chemical signals that allow us to sniff out optimal mates . and as we explained in a previous episode about sex appeal , people tend to be attracted to mates with different immune system make-up than their own , and swapping spit is a great way to get a snoot full of pheromones to see if your potential mate is a good fit . all that history and biology aside , kissing is probably one of those things that its best not to really over-think . personally , i 'd rather just remember that nervous , sweaty magic of my first kiss than picture a mouthful of pre-chewed meat coming at me . thanks for watching this episode of scishow , you can find us on facebook , twitter , or down in the comments below , and if you want to keep getting smarter with us here at scishow , you can go to youtube.com/scishow and subscribe .

to a guinness book title , apparently . by the middle ages , people all over europe were swapping spit , and by the 19th century we actually gave the study of kissing the decidedly un-sexy title of philematology . rrrr .

what is the study of kissing called ?

michael jordan once said , `` i do n't know whether i 'll fly or not . i know that when i 'm in the air sometimes i feel like i do n't ever have to come down . '' but thanks to isaac newton , we know that what goes up must eventually come down . in fact , the human limit on a flat surface for hang time , or the time from when your feet leave the ground to when they touch down again , is only about one second , and , yes , that even includes his airness , whose infamous dunk from the free throw line has been calculated at .92 seconds . and , of course , gravity is what 's making it so hard to stay in the air longer . earth 's gravity pulls all nearby objects towards the planet 's surface , accelerating them at 9.8 meters per second squared . as soon as you jump , gravity is already pulling you back down . using what we know about gravity , we can derive a fairly simple equation that models hang time . this equation states that the height of a falling object above a surface is equal to the object 's initial height from the surface plus its initial velocity multiplied by how many seconds it 's been in the air , plus half of the gravitational acceleration multiplied by the square of the number of seconds spent in the air . now we can use this equation to model mj 's free throw dunk . say mj starts , as one does , at zero meters off the ground , and jumps with an initial vertical velocity of 4.51 meters per second . let 's see what happens if we model this equation on a coordinate grid . since the formula is quadratic , the relationship between height and time spent in the air has the shape of a parabola . so what does it tell us about mj 's dunk ? well , the parabola 's vertex shows us his maximum height off the ground at 1.038 meters , and the x-intercepts tell us when he took off and when he landed , with the difference being the hang time . it looks like earth 's gravity makes it pretty hard for even mj to get some solid hang time . but what if he were playing an away game somewhere else , somewhere far ? well , the gravitational acceleration on our nearest planetary neighbor , venus , is 8.87 meters per second squared , pretty similar to earth 's . if michael jumped here with the same force as he did back on earth , he would be able to get more than a meter off the ground , giving him a hang time of a little over one second . the competition on jupiter with its gravitational pull of 24.92 meters per second squared would be much less entertaining . here , michael would n't even get a half meter off the ground , and would remain airborne a mere .41 seconds . but a game on the moon would be quite spectacular . mj could take off from behind half court , jumping over six meters high , and his hang time of over five and half seconds , would be long enough for anyone to believe he could fly .

let 's see what happens if we model this equation on a coordinate grid . since the formula is quadratic , the relationship between height and time spent in the air has the shape of a parabola . so what does it tell us about mj 's dunk ?

what type of function is the relationship between height and hang time ?

a handful of species on earth share a seemingly mysterious trait : a menstrual cycle . we 're one of the select few . monkeys , apes , bats , humans , and possibly elephant shrews are the only mammals on earth that menstruate . we also do it more than any other animal , even though its a waste of nutrients and can be a physical inconvenience . so where 's the sense in this uncommon biological process ? the answer begins with pregnancy . during this process , the body 's resources are cleverly used to shape a suitable environment for a fetus , creating an internal haven for a mother to nurture her growing child . in this respect , pregnancy is awe-inspiring , but that 's only half the story . the other half reveals that pregnancy places a mother and her child at odds . as for all living creatures , the human body evolved to promote the spread of its genes . for the mother , that means she should try to provide equally for all her offspring . but a mother and her fetus do n't share exactly the same genes . the fetus inherits genes from its father , as well , and those genes can promote their own survival by extracting more than their fair share of resources from the mother . this evolutionary conflict of interests places a woman and her unborn child in a biological tug-of-war that plays out inside the womb . one factor contributing to this internal tussle is the placenta , the fetal organ that connects to the mother 's blood supply and nourishes the fetus while it grows . in most mammals , the placenta is confined behind a barrier of maternal cells . this barrier lets the mother control the supply of nutrients to the fetus . but in humans and a few other species , the placenta actually penetrates right into the mother 's circulatory system to directly access her blood stream . through its placenta , the fetus pumps the mother 's arteries with hormones that keep them open to provide a permanent flow of nutrient-rich blood . a fetus with such unrestricted access can manufacture hormones to increase the mother 's blood sugar , dilate her arteries , and inflate her blood pressure . most mammal mothers can expel or reabsorb embryos if required , but in humans , once the fetus is connected to the blood supply , severing that connection can result in hemorrhage . if the fetus develops poorly or dies , the mother 's health is endangered . as it grows , a fetus 's ongoing need for resources can cause intense fatigue , high blood pressure , and conditions like diabetes and preeclampsia . because of these risks , pregnancy is always a huge , and sometimes dangerous , investment . so it makes sense that the body should screen embryos carefully to find out which ones are worth the challenge . this is where menstruation fits in . pregnancy starts with a process called implantation , where the embryo embeds itself in the endometrium that lines the uterus . the endometrium evolved to make implantation difficult so that only the healthy embryos could survive . but in doing so , it also selected for the most vigorously invasive embryos , creating an evolutionary feedback loop . the embryo engages in a complex , exquisitely timed hormonal dialogue that transforms the endometrium to allow implantation . what happens when an embryo fails the test ? it might still manage to attach , or even get partly through the endometrium . as it slowly dies , it could leave its mother vulnerable to infection , and all the time , it may be emitting hormonal signals that disrupt her tissues . the body avoids this problem by simply removing every possible risk . each time ovulation does n't result in a healthy pregnancy , the womb gets rid of its endometrial lining , along with any unfertilized eggs , sick , dying , or dead embryos . that protective process is known as menstruation , leading to the period . this biological trait , bizarre as it may be , sets us on course for the continuation of the human race .

each time ovulation does n't result in a healthy pregnancy , the womb gets rid of its endometrial lining , along with any unfertilized eggs , sick , dying , or dead embryos . that protective process is known as menstruation , leading to the period . this biological trait , bizarre as it may be , sets us on course for the continuation of the human race .

explain this statement : menstruation is a protective mechanism for the female .

translator : bedirhan cinar my wife is pregnant right now with our first child , and when people see her with her big baby bump , the first question people ask , almost without fail , is , `` is it a boy or is it a girl ? '' now , there are some assumptions behind that question that we take for granted because of our familiarity with our own human biology . for human babies , we take it for granted that there 's a 50/50 chance of either answer , boy or girl . but why is it that way ? well , the answer depends on the sex determination system that has evolved for our species . you see , for most mammals , the sex of a baby is determined genetically with the xy chromosome system . mammals have a pair of sex chromosomes , one passed down from mom , and one from dad . a pair of x 's gives us a girl , and an x and a y together gives us a boy . since females only have x 's to pass on in their egg cells , and males can give either an x or a y in their sperm cells , the sex is determined by the father and the chance of producing a male or a female is 50/50 . this system has worked well for mammals , but throughout the tree of life , we can see other systems that have worked just as well for other animals . there are other groups of animals that also have genetic sex determination , but their systems can be pretty different from ours . birds and some reptiles have their sex genetically determined , but instead of the sex being determined by dad , their sex is determined by mom . in those groups , a pair of z sex chromosomes produces a male , so these males only have z 's to give . however , in these animals , one z and one w chromosome together , as a pair , produces a female . in this system , the chance of a male or a female is still 50/50 , it just depends on whether mom puts a z or a w into her egg . certain groups have taken genetic sex determination in completely other directions . ants , for example , have one of the most interesting systems for determining sex , and because of it , if you are a male ant , you do not have a father . in an ant colony , there are dramatic divisions of labor . there are soldiers that defend the colony , there are workers that collect food , clean the nest and care for the young , and there 's a queen and a small group of male reproductives . now , the queen will mate and then store sperm from the males . and this is where the system gets really interesting . if the queen uses the stored sperm to fertilize an egg , then that egg will grow up to become female . however , if she lays an egg without fertilizing it , then that egg will still grow up to be an ant , but it will always be a male . so you see , it 's impossible for male ants to have fathers . and male ants live their life like this , with only one copy of every gene , much like a walking sex cell . this system is called a haplodiploid system , and we see it not only in ants , but also in other highly social insects like bees and wasps . since our own sex is determined by genes , and we do know of these other animals that have their sex determined by genes , it 's easy to assume that for all animals the sex of their babies still must be determined by genetics . however , for some animals , the question of whether it will be a boy or a girl has nothing to do with genes at all , and it can depend on something like the weather . these are animals like alligators and most turtles . in these animals , the sex of an embryo in a developing egg is determined by the temperature . in these species , the sex of the baby is not yet determined when the egg is laid , and it remains undetermined until sometime in the middle of the overall development period , when a critical time is reached . and during this time , the sex is completely determined by temperature in the nest . in painted turtles , for example , warm temperatures above the critical temperature will produce females within the eggs , and cool temperatures will produce a male . i 'm not sure who came up with this mnemonic , but you can remember that when it comes to painted turtles , they are all hot chicks and cool dudes . for some tropical fish , the question of will it be a boy or will it be a girl is n't settled until even later in life . you see , clownfish all start out their lives as males , however , as they mature , they become female . they also spend their lives in small groups with a strict dominance hierarchy where only the most dominant male and female reproduce . and amazingly , if the dominant female in the group dies , the largest and most dominant male will then quickly become female and take her place , and all of the other males will move up one rank in the hierarchy . in another very different ocean animal , the green spoonworm , the sex of the babies is determined by a completely different aspect of the environment . for this species , it is simply a matter of where a larva happens to randomly fall on the sea floor . if a larva lands on the open sea floor , then it will become a female . but if it lands on top of a female , then it will become a male . so for some species , the question of boy or girl is answered by genetics . for others , it 's answered by the environment . and for others still , they do n't even bother with the question at all . take whiptail lizards , for example . for those desert lizards , the answer is easy . it 's a girl . it 's always a girl . they are a nearly all-female species , and although they still lay eggs , these eggs hatch out female clones of themselves . so will it be a girl or will it be a boy ? throughout the entire animal kingdom , it does really all depend on the system of sex determination . for humans , that system is a genetic xy system . and for me and my wife , we found out it 's going to be a baby boy . ( kiss )

this system has worked well for mammals , but throughout the tree of life , we can see other systems that have worked just as well for other animals . there are other groups of animals that also have genetic sex determination , but their systems can be pretty different from ours . birds and some reptiles have their sex genetically determined , but instead of the sex being determined by dad , their sex is determined by mom . in those groups , a pair of z sex chromosomes produces a male , so these males only have z 's to give .

the sex determination systems for most mammals and birds are similar in that , for both groups sex is determined by

he was one of the most influential figures of the 20th century , forever changing the course of one of the world 's largest countries . but was he a hero who toppled an oppressive tyranny or a villain who replaced it with another ? it 's time to put lenin on the stand in history vs. lenin . `` order , order , hmm . now , was n't it your fault that the band broke up ? '' `` your honor , this is vladimir ilyich ulyanov , aka lenin , the rabblerouser who helped overthrow the russian tsar nicholas ii in 1917 and founded the soviet union , one of the worst dictatorships of the 20th century . '' `` ohh . '' `` the tsar was a bloody tyrant under whom the masses toiled in slavery . '' `` this is rubbish . serfdom had already been abolished in 1861 . '' `` and replaced by something worse . the factory bosses treated the people far worse than their former feudal landlords . and unlike the landlords , they were always there . russian workers toiled for eleven hours a day and were the lowest paid in all of europe . '' `` but tsar nicholas made laws to protect the workers . '' `` he reluctantly did the bare minimum to avert revolution , and even there , he failed . remember what happened in 1905 after his troops fired on peaceful petitioners ? '' `` yes , and the tsar ended the rebellion by introducing a constitution and an elected parliament , the duma . '' `` while retaining absolute power and dissolving them whenever he wanted . '' `` perhaps there would 've been more reforms in due time if radicals , like lenin , were n't always stirring up trouble . '' `` your honor , lenin had seen his older brother aleksandr executed by the previous tsar for revolutionary activity , and even after the reforms , nicholas continued the same mass repression and executions , as well as the unpopular involvement in world war i , that cost russia so many lives and resources . '' `` hm , this tsar does n't sound like such a capital fellow . '' `` your honor , maybe nicholas ii did doom himself with bad decisions , but lenin deserves no credit for this . when the february 1917 uprisings finally forced the tsar to abdicate , lenin was still exiled in switzerland . '' `` hm , so who came to power ? '' `` the duma formed a provisional government , led by alexander kerensky , an incompetent bourgeois failure . he even launched another failed offensive in the war , where russia had already lost so much , instead of ending it like the people wanted . '' `` it was a constitutional social democratic government , the most progressive of its time . and it could have succeeded eventually if lenin had n't returned in april , sent by the germans to undermine the russian war effort and instigate riots . '' `` such slander ! the july days were a spontaneous and justified reaction against the government 's failures . and kerensky showed his true colors when he blamed lenin and arrested and outlawed his bolshevik party , forcing him to flee into exile again . some democracy ! it 's a good thing the government collapsed under their own incompetence and greed when they tried to stage a military coup then had to ask the bolsheviks for help when it backfired . after that , all lenin had to do was return in october and take charge . the government was peacefully overthrown overnight . '' `` but what the bolsheviks did after gaining power was n't very peaceful . how many people did they execute without trial ? and was it really necessary to murder the tsar 's entire family , even the children ? '' `` russia was being attacked by foreign imperialists , trying to restore the tsar . any royal heir that was rescued would be recognized as ruler by foreign governments . it would 've been the end of everything the people had fought so hard to achieve . besides , lenin may not have given the order . '' `` but it was not only imperialists that the bolsheviks killed . what about the purges and executions of other socialist and anarchist parties , their old allies ? what about the tambov rebellion , where peasants , resisting grain confiscation , were killed with poison gas ? or sending the army to crush the workers in kronstadt , who were demanding democratic self-management ? was this still fighting for the people ? '' `` yes ! the measures were difficult , but it was a difficult time . the new government needed to secure itself while being attacked from all sides , so that the socialist order could be established . '' `` and what good came of this socialist order ? even after the civil war was won , there were famines , repression and millions executed or sent to die in camps , while lenin 's successor stalin established a cult of personality and absolute power . '' `` that was n't the plan . lenin never cared for personal gains , even his enemies admitted that he fully believed in his cause , living modestly and working tirelessly from his student days until his too early death . he saw how power-hungry stalin was and tried to warn the party , but it was too late . '' `` and the decades of totalitarianism that followed after ? '' `` you could call it that , but it was lenin 's efforts that changed russia in a few decades from a backward and undeveloped monarchy full of illiterate peasants to a modern , industrial superpower , with one of the world 's best educated populations , unprecedented opportunities for women , and some of the most important scientific advancements of the century . life may not have been luxurious , but nearly everyone had a roof over their head and food on their plate , which few countries have achieved . '' `` but these advances could still have happened , even without lenin and the repressive regime he established . '' `` yes , and i could 've been a famous rock and roll singer . but how would i have sounded ? '' we can never be sure how things could 've unfolded if different people were in power or different decisions were made , but to avoid the mistakes of the past , we must always be willing to put historical figures on trial .

he was one of the most influential figures of the 20th century , forever changing the course of one of the world 's largest countries . but was he a hero who toppled an oppressive tyranny or a villain who replaced it with another ?

what were some causes behind discontent with czarist rule ?

( music ) quick ! what 's common between beef burgers , baseball training and auto mufflers ? tough question . let 's ask it another way . what 's the common factor between mcdonald 's , d-bat and meineke ? you may know the answer if , along with a big mac , you 've absorbed a fragment of the romantic story of ray kroc . he 's the salesman that created what became the world 's biggest fast food chain . he did it by making a deal with a couple of men called the mcdonalds . brothers they were , owners of a small restaurant chain , and the deal was , he could use their brand name and their methods . then he invited small entrepreneurs to open mcdonald 's , that they 'd run as operators , with an ownership state . very different than the business model where mom and pop stores have full ownership , but no similar support . all the examples in my opening question are a franchise operation . kroc is sometimes credited with inventing franchising , and so is isaac singer , the sewing machine magnate . not so . the real genesis of franchising was not in stitches or beef , it was in beauty . martha matilda harper was a canadian-born maid . she made the beds , cleaned house , did the shopping . in the employment of a doctor 's family in ontario , she acquired a secret formula for shampoo , one more scientifically based than the quackeries advertized every day in the newspapers . the kindly doctor also taught the maturing young woman the elements of physiology . martha had a secret ambition to go along with the secret formula : a determination to run her own business . by 1888 , serving as a maid in rochester , new york , she saved enough money -- 360 dollars -- to think of opening a public hairdressing salon . but before she could realize her dream , two blows fell . she became sick , and collapsed from exhaustion . mrs. helen smith , a healing practitioner of the christian science faith , was summoned to her bedside . the two women prayed , and martha recovered . no sooner was she better then she was told , `` oh no , you ca n't rent the place you 've eyed . '' you see , her venture was to be the first public hairdressing salon . a woman in business was shocking enough then . only 17 percent of the workforce in 1890 was female , but a woman carrying out hairdressing and skincare in a public place ? why , it was sure to invite a scandal . martha spent some of her savings on a lawyer , and won her case . she proudly displayed on the door of her new her salon a photograph of the barely five-foot martha as rapunzel , with hair down to her feet , but glowing with good health . her sickness , too , had proved a boon . her ambition was now propelled by christian science values . the harper method , as she came to call her services , was as much about servicing the soul as it was about cutting hair . in the therapeutic serenity of her salon , she taught that every person could glow with the kind of beauty she had , if spiritually whole and physically obedient to what she called `` the laws of cleanliness , nourishment , exercise and breathing . '' she was very practical about it . she even designed the first reclining shampoo chair , though she neglected to patent the invention . martha 's salon was a huge success . celebrities came from out of town to experience the harper method . they enjoyed the service so much that they urged her to set up a salon in their cities . and this is where martha 's ethical sense inspired her crowning innovation . instead of commissioning agents , as other innovators had done , from 1891 , she installed working-class women just like herself in salons exactly like hers , dedicated to her philosophy and her products . but these new employees were not provided a salary by martha . the women in what became a satellite network of 500 salons in america , and then europe and central america and asia , actually owned the harper 's salons . what was good enough in the nineteenth century for suffragette campaigners like susan b. anthony and was good enough in the twentieth century for woodrow wilson , calvin and grace coolidge , jacqueline kennedy , helen hayes and ladybird johnson must be good enough for the rest of the world . today , only the harper method founder 's shop remains in rochester , new york , but martha 's legacy is manifold . her health and beauty treatments have been copied , and her business model is dominant . in fact , half of retail sales in america are through martha harper 's franchising idea . so the next time you enjoy a mcdonald 's hamburger or a good night 's rest at a days inn , think of martha . because these franchises might not be the same without her inventing the model , over a century ago .

her health and beauty treatments have been copied , and her business model is dominant . in fact , half of retail sales in america are through martha harper 's franchising idea . so the next time you enjoy a mcdonald 's hamburger or a good night 's rest at a days inn , think of martha .

how did harper provide a distinctive experience for her customers , franchise owners , and employees ? what are her other legacies , in addition to her franchising idea ?

before the creation of humanity , the greek gods won a great battle against a race of giants called the titans . most titans were destroyed or driven to the eternal hell of tartarus . but the titan prometheus , whose name means foresight , persuaded his brother epimetheus to fight with him on the side of the gods . as thanks , zeus entrusted the brothers with the task of creating all living things . epimetheus was to distribute the gifts of the gods among the creatures . to some , he gave flight ; to others , the ability to move through water or race through grass . he gave the beasts glittering scales , soft fur , and sharp claws . meanwhile , prometheus shaped the first humans out of mud . he formed them in the image of the gods , but zeus decreed they were too remain mortal and worship the inhabitants of mount olympus from below . zeus deemed humans subservient creatures vulnerable to the elements and dependent on the gods for protection . however , prometheus envisioned his crude creations with a greater purpose . so when zeus asked him to decide how sacrifices would be made , the wily prometheus planned a trick that would give humans some advantage . he killed a bull and divided it into two parts to present to zeus . on one side , he concealed the succulent flesh and skin under the unappealing belly of the animal . on the other , he hid the bones under a thick layer of fat . when zeus chose the seemingly best portion for himself , he was outraged at prometheus 's deception . fuming , zeus forbade the use of fire on earth , whether to cook meat or for any other purpose . but prometheus refused to see his creations denied this resource . and so , he scaled mount olympus to steal fire from the workshop of hephaestus and athena . he hid the flames in a hollow fennel stalk and brought it safely down to the people . this gave them the power to harness nature for their own benefit and ultimately dominate the natural order . with fire , humans could care for themselves with food and warmth . but they could also forge weapons and wage war . prometheus 's flames acted as a catalyst for the rapid progression of civilization . when zeus looked down at this scene , he realized what had happened . prometheus had once again wounded his pride and subverted his authority . furious , zeus imposed a brutal punishment . prometheus was to be chained to a cliff for eternity . each day , he would be visited by a vulture who would tear out his liver and each night his liver would grow back to be attacked again in the morning . although prometheus remained in perpetual agony , he never expressed regret at his act of rebellion . his resilience in the face of oppression made him a beloved figure in mythology . he was also celebrated for his mischievous and inquisitive spirit , and for the knowledge , progress , and power he brought to human hands . he 's also a recurring figure in art and literature . in percy bysshe shelley 's lyrical drama `` prometheus unbound , '' the author imagines prometheus as a romantic hero who escapes and continues to spread empathy and knowledge . of his protagonist , shelley wrote , `` prometheus is the type of the highest perfection of moral and intellectual nature , impelled by the purest and the truest motives to the best and noblest ends . '' his wife mary envisaged prometheus as a more cautionary figure and subtitled her novel `` frankenstein : the modern prometheus . '' this suggests the damage of corrupting the natural order and remains relevant to the ethical questions surrounding science and technology today . as hero , rebel , or trickster , prometheus remains a symbol of our capacity to capture the powers of nature , and ultimately , he reminds us of the potential of individual acts to ignite the world .

he formed them in the image of the gods , but zeus decreed they were too remain mortal and worship the inhabitants of mount olympus from below . zeus deemed humans subservient creatures vulnerable to the elements and dependent on the gods for protection . however , prometheus envisioned his crude creations with a greater purpose . so when zeus asked him to decide how sacrifices would be made , the wily prometheus planned a trick that would give humans some advantage . he killed a bull and divided it into two parts to present to zeus .

zeus wanted the humans prometheus had made :

austrian physicist erwin schrödinger is one of the founders of quantum mechanics , but he 's most famous for something he never actually did : a thought experiment involving a cat . he imagined taking a cat and placing it in a sealed box with a device that had a 50 % chance of killing the cat in the next hour . at the end of that hour , he asked , `` what is the state of the cat ? '' common sense suggests that the cat is either alive or dead , but schrödinger pointed out that according to quantum physics , at the instant before the box is opened , the cat is equal parts alive and dead , at the same time . it 's only when the box is opened that we see a single definite state . until then , the cat is a blur of probability , half one thing and half the other . this seems absurd , which was schrödinger 's point . he found quantum physics so philosophically disturbing , that he abandoned the theory he had helped make and turned to writing about biology . as absurd as it may seem , though , schrödinger 's cat is very real . in fact , it 's essential . if it were n't possible for quantum objects to be in two states at once , the computer you 're using to watch this could n't exist . the quantum phenomenon of superposition is a consequence of the dual particle and wave nature of everything . in order for an object to have a wavelength , it must extend over some region of space , which means it occupies many positions at the same time . the wavelength of an object limited to a small region of space ca n't be perfectly defined , though . so it exists in many different wavelengths at the same time . we do n't see these wave properties for everyday objects because the wavelength decreases as the momentum increases . and a cat is relatively big and heavy . if we took a single atom and blew it up to the size of the solar system , the wavelength of a cat running from a physicist would be as small as an atom within that solar system . that 's far too small to detect , so we 'll never see wave behavior from a cat . a tiny particle , like an electron , though , can show dramatic evidence of its dual nature . if we shoot electrons one at a time at a set of two narrow slits cut in a barrier , each electron on the far side is detected at a single place at a specific instant , like a particle . but if you repeat this experiment many times , keeping track of all the individual detections , you 'll see them trace out a pattern that 's characteristic of wave behavior : a set of stripes - regions with many electrons separated by regions where there are none at all . block one of the slits and the stripes go away . this shows that the pattern is a result of each electron going through both slits at the same time . a single electron is n't choosing to go left or right but left and right simultaneously . this superposition of states also leads to modern technology . an electron near the nucleus of an atom exists in a spread out , wave-like orbit . bring two atoms close together , and the electrons do n't need to choose just one atom but are shared between them . this is how some chemical bonds form . an electron in a molecule is n't on just atom a or atom b , but a+ b . as you add more atoms , the electrons spread out more , shared between vast numbers of atoms at the same time . the electrons in a solid are n't bound to a particular atom but shared among all of them , extending over a large range of space . this gigantic superposition of states determines the ways electrons move through the material , whether it 's a conductor or an insulator or a semiconductor . understanding how electrons are shared among atoms allows us to precisely control the properties of semiconductor materials , like silicon . combining different semiconductors in the right way allows us to make transistors on a tiny scale , millions on a single computer chip . those chips and their spread out electrons power the computer you 're using to watch this video . an old joke says that the internet exists to allow the sharing of cat videos . at a very deep level , though , the internet owes its existance to an austrian physicist and his imaginary cat .

an electron in a molecule is n't on just atom a or atom b , but a+ b . as you add more atoms , the electrons spread out more , shared between vast numbers of atoms at the same time . the electrons in a solid are n't bound to a particular atom but shared among all of them , extending over a large range of space . this gigantic superposition of states determines the ways electrons move through the material , whether it 's a conductor or an insulator or a semiconductor .

the behavior of electrons moving through solid objects is determined by :

almost 2000 years ago , the roman philosopher seneca peered at his book through a glass of water . suddenly , the text below was transformed . the words magically became clear . but it was n't until a millennium later that that same principle would be used to create the earliest glasses . today , glasses can help millions of people with poor vision due to uncorrected refractive errors . the key to understanding what that means lies with the term refraction , the ability of a transparent medium , like glass , water , or the eye to change the direction of light passing through it . the eye has two main refractive surfaces : the cornea and the lens . ideally , these surfaces work together to refract light in a way that accurately focuses light onto the retina , the layer of light-sensitive tissue at the back of the eye that works with the brain to give rise to vision . but many people develop refractive errors , either during childhood as their eyes are growing , or in later life as their eyes age . imperfections in the cornea and lens cause refracted light to be focused in front of or behind the retina , making images appear blurry . people with refractive errors can still see color , movement , and light , but the details of what they 're looking at are out of focus . people experience refractive errors in different ways , owing to differences in their eyes . in some , light refracts too much , and in others , too little . eyes with a focal point in front of the retina are called myopic , or short-sighted . they can see close objects clearly , but those far away are out of focus . but when the focus point is behind the retina , people are hyperopic , or long-sighted . for them , objects close up are unfocused , but distant objects are crystal clear . finally , some people have a cornea with a non-spherical shape that causes astigmatism , a form of out-of-focus vision that makes all objects seem blurred , whether close or far . as we age , our eyes face new challenges . when we 're young , the lens of the eye is flexible and can change shape to bring images into focus , something called accommodation . this keeps objects in focus when we shift our gaze from far to near . but as we get older , the lens becomes less flexible , and ca n't change shape when we want to look at near objects . this is called presbyopia , and it affects adults starting around the age of 40 years . myopia , hyperopia , astigmatism , and presbyopia . each of these is a refractive error . nowadays we can fix them all with glasses or contact lenses , which work by refocusing light so it strikes the retina precisely . it 's even possible to correct vision with surgery using lasers that change the shape of the cornea and alter its refractive properties . but glasses remain the most popular . by using carefully crafted lenses to steer light to exactly the right spot on the retina , a person 's clear vision can be restored . we 've come a long way since seneca 's discovery and the crude glasses of yesteryear . in 1727 , a british optician named edward scarlett developed the modern style of glasses which are kept in place with arms which hook over each ear . today 's glasses take their inspiration from that design , but they 're also much more precise and personal . each pair is tailored for an individual to bring out their unique powers of sight . so if you 're one of the 500 million people with a problem with close or far vision , or both , there 's a pair of glasses out there waiting to reveal a whole new world that 's hiding in plain view .

in some , light refracts too much , and in others , too little . eyes with a focal point in front of the retina are called myopic , or short-sighted . they can see close objects clearly , but those far away are out of focus .

which condition can be described as over-refracting light so that the focal point is in front of the retina ?

here at scishow hq we have a little food area for the employees - sometimes there are donuts . sometimes there are nuts . sometimes dried mango . but the one thing that never sticks around and is gone as soon as we can buy it is the wonderful , beautiful , noble banana . unfortunately for us , they may not be around forever . [ intro music ] first , the good : bananas are healthy , packed with nutrition and energy , they fit in your hand and give nice little cues when they 're perfectly ripe , and are easy to peel and eat ; shocking statistic , the banana is wal-mart 's number one selling item . not the potato chip , not coca-cola , not fifty shades of grey , bananas . they appear to be so perfect for human consumption that kirk cameron attempted to use them to prove the existence of god . of course this banana was not created by god , or really even nature . bananas , at least the ones that you see at the store , were created by people . do n't get me wrong , there are wild banana plants - lots of them - they 're native to south and southeast asia , and there are dozens of species and thousands of varieties . they 're just not the ones we eat . some those species , as you might suspect , have seeds , 'cause that 's what fruits are , they 're fleshy bodies containing seeds . so you might wonder , why have you never eaten a banana seed ? well , you have ... kinda . in cultivated bananas the seeds have pretty much stopped existing . if you look closely you can see tiny black specks . those are all that 's left , and they 're not fertile seeds . if you plant them , nothing grows . today 's bananas are sterile mutants . i 'm not trying to be mean , that 's just the truth . unless you were alive in the 1960 's ( hats off to all those older scishow viewers out there ) every banana you have ever eaten was pretty much genetically identical . this is a cavendish , the virtually seedless variety that we all eat today , but it was n't always our banana of choice . until the 1960s , everyone was eating the same banana , it was just a different banana - the gros michel , a bigger , sweeter fruit with thicker skin . you might notice that banana flavored things do n't really taste like bananas . well they do - they taste like the gros michel . the genetic monotony of the gros michel crop was its undoing . a fungicide resistant pathogen called the panama disease began infecting gros michel crop . by the time growers understood how vulnerable their crops were , the gros michel variety was all but extinct . the entire banana industry had to be retooled for the cavendish . since they 're seedless , the only way to reproduce them is to transplant part of the plant stem , and for the last 50 years we 've been good with the cavendish , 'cause it 's more resistant to the panama disease . however somewhat terrifyingly a strain of panama disease that affects the cavendish strain that we all eat has been identified . a global monoculture of genetically identical individuals is a beautiful sight to a pathogen . the fungus only has to figure out how to infect and destroy a single individual , and suddenly there is no diversity to stop it , or even slow it down . that 's led to a lot of scientists worrying about or even predicting the outright demise of the cavendish . this wonderful most popular of fruits might completely cease existence . the good news is we now have a much better understanding of genetics , epidemics , fungi , and pathology . scientists and growers have already taken steps to protect the cavendish . some growers are creating genetically different bananas that might replace the cavendish crop if it fails , while scientists are attempting to genetically engineer cavendish plants with immunity to panama disease . plus we learned a lot from the gros michel debacle . infected fields are quickly being destroyed and new crops are grown from pathogen-free lab-grown plant stock . so thanks to the people who work tirelessly to grow and harvest bananas and bring them to us so that we can offer them inexpensively to our employees , and thanks to the growers and scientists working tirelessly to make sure that they do n't go the way of the gros michel . thanks for watching this episode of scishow , if you have any questions , comments or suggestions for us , you can find us on facebook , twitter or in the comments below , and if you want to continue getting smarter this year at scishow , you can go to youtube.com/scishow and subscribe . [ banana eating noises ] [ music ]

however somewhat terrifyingly a strain of panama disease that affects the cavendish strain that we all eat has been identified . a global monoculture of genetically identical individuals is a beautiful sight to a pathogen . the fungus only has to figure out how to infect and destroy a single individual , and suddenly there is no diversity to stop it , or even slow it down .

6 . `` a global monoculture of genetically identical fruit ( like banana ) is a beautiful sight to a pathogen . '' explain why .

whether you 're watching an entire netflix series in one sitting , or playing the newest zelda from start to finish , most of us spend a significant time in front of screens . but is binge watching bad for your brain and body ? in the past , it may have actually been deadly . in 1967 , an error in manufacturing led to the selling of tvs which emitted harmful x-rays with radiation levels 100,000 times higher than what is considered safe today . but even modern televisions cause a strain on your eyes . under normal circumstances , humans blink around 18 times per minute , but when staring at a screen , this rate decreases drastically causing sore and tired eyes . fortunately , these symptoms are generally short-term . in children , however , simply spending extended hours indoors can have developmental effects . a condition called myopia , where the eyes can not focus properly , is seen much more frequently in children who spend more time inside . not only are you constantly forced to focus on nearby objects as opposed to far off landscapes and distances , but scientists believe that the sun itself may actually play a role in healthy eye regulation . and while tv may feel like a nice way to relax your body and brain , that may not always be a good thing . not only is a sedentary lifestyle a major contributor to obesity , but studies have shown that people who watch less tv tend to burn more calories , even if they are n't doing more physical activity . simply doing more mentally rigorous tasks like reading , playing a board game , or simple household activities , requires more energy and burns more calories . and if you want to truly relax , tv before bed may also be hurting you . studies have shown that it may actually reduce the hours of quality sleep , contributing to chronic sleep debt . it may also affect other bedtime activities . researchers have found that those who watch more than 20 hours of tv a week have , on average , a 44 % reduction in sperm . but perhaps the most significant findings relate directly to your life span . not only is there a documented correlation between tv viewing time and risk for diabetes and heart disease , but as shocking as it may be , multiple studies have found a correlation between tv viewing time and all causes of death . one study concluded that every hour spent in front of the tv may cut as much as 22 minutes off your life . of course , correlation does not equal causation , and you can have a healthy relationship with tv if consumed in moderation like anything else . at the root of many of these claims is the physical inactivity associated with prolonged tv watching . ultimately , the more you move , the more you live . be sure to check out our newest videos by clicking the screen or using the links in the description , and subscribe for more weekly science videos every thursday .

and while tv may feel like a nice way to relax your body and brain , that may not always be a good thing . not only is a sedentary lifestyle a major contributor to obesity , but studies have shown that people who watch less tv tend to burn more calories , even if they are n't doing more physical activity . simply doing more mentally rigorous tasks like reading , playing a board game , or simple household activities , requires more energy and burns more calories .

people who watch less tv burn more calories , even if they 're just reading a book . why ?

we 've decided to make a new video about aluminium because it 's a long time over five years since we made the first one and ... we did n't say very much . aluminium is a surprisingly abundand element . if you look at this periodic table here , where the area of the different elements gives you a rough idea of the abundance you can see that aluminium is one of the most abundant metals up there with sodium , magnesium and calcium . more aluminum than potassium ; about the same or perhaps even more than iron . we 're never going to run out of aluminium . the problem with aluminium is that you do n't find aluminium metal in nature as a metal . it 's always tied up with other compounds ; mostly with oxygen , in clays . you know what clays are , the sort of muddy stuff that you get stuck on your shoes when it 's raining . to get the aluminium out of the clay , that is , to break the aluminium/oxygen bonds , which are very strong ; requires a lot of energy , which comes from electricity . so , making aluminium is very energy intensive . that 's why people like to recycle aluminium because once you 've got it , it 's worth preserving ; but it 's fantastically important because aluminium is a very light metal . and it 's often used as an alloy because the aluminium alloys are stronger than the aluminium itself , so , if you 're using it for aircraft or some other use like that where you want to combine lightness with strength then the stronger you can make it , the better . but when it was first made , in the 19th century , isolated as a metal it was terrifically valuable and there are stories of the french emperor serving his honored guests with aluminium plates or aluminium cutlery while the less important people had silver or gold ; but those times have passed and now you can get cupcakes and things like that surrounded by foil of aluminium . aluminium is a very good metal for making things because it has a very thin coating of aluminium oxide on the surface which prevents it [ from ] reacting with things . but as soon as that coating goes it becomes very reactive . alfred worden : hadley base , do you read houston ? david scott : yeah . now , 5 by , joe . worden : okay . worden : and i guess we 're standing by for your high-gain alignment per the checklist . scott : okay , stand by . you may have seen our video where we put copper chloride in one of these cupcake holders ... [ first of all i 'm going to dissolve some up and make a fairly concentrated solution . i 'm going to place this here . ] ... and what came out was this , or rather the copper chloride came out through the hole . [ it starts boiling really quite nicely . now , imagine i was doing this for my children who were quite small at that time , and ... voosh ! ] and the aluminium was completely dissolved up forming aluminium chloride and copper metal . in my own research , aluminium is quite important ; quite a lot of our equipment uses aluminium . not so much for the high pressure tubing that we use because quite a lot of my reaserch involves high pressures but we use it for the metal blocks that we put round the tubing so that we can heat it up . aluminium has a good electrical conductivity , and it 's also easy to machine . this is a piece of equipment here where we have a tube going down the middle . you can see the diameter of the tube here . around it is an aluminium block and an electrical heater . now , this particular case there was an accident , or a mishap , because the thermocouple that was controlling the temperature of this fell out . so , the heater got hotter and hotter , and eventually , the aluminium melted and poured down here . and i think this is really beautiful . well , fortunately , i was not in the lab or i would 've got very angry with my students but i think when it happened it was quite exciting ; this would have been glowing almost red because the melting point of aluminium is around 500 degrees centigrade . but then once it formed originally it was very shiny but quickly , it again developed the surface layer of aluminium oxide . if you have fine particles of aluminium and blow them into a flame ... ... then they will burn quite spectacularly and you form aluminium oxide . now , on the face of it , aluminium oxide sounds a rather boring compound but it 's really very useful and we use it quite a lot in our research in all sorts of different ways . it looks like a white powder . not very exciting . but in our group this aluminium oxide has been a fantastic catalyst all sorts of reactions that we did n't expect have gone with this material . my students keep it in a bottle almost like a magic catalyst and i 've only been given a little to show you . it acts as a solid acid which can be used at very high temperature and will get various acid-catalyzed reactions of organic compounds . it will make ethers , we have made various alkynes and a whole series of different compounds and my students still use it very much . if you melt the aluminium oxide , which we ca n't do here but can be done industrially , you can make single crystals which are transparent like glass and then you can grow a single crystal tube , like this one , which because it 's a single crystal , it 's terrifically strong . it 's the defects that make something weak and so if you have just one crystal there are no defects and so it 's very strong . so you can put a very high pressure inside this tube without it blowing up . brady : but you could make that tube out of metal , professor . professor : but , if you have a metal then you ca n't see what 's going on inside , and we 're using these tubes for photochemical reactions . so , we take a light like this , and shine it on the chemicals going through the tube under high pressure and we can convert one chemical into another . we can do this very efficiently because the light is absorbed by the molecules that we want to react and so we dont waste the energy on everything else . and using leds , which are a very efficient light source , you can get a process that is very energy efficient and it all depends on having this sapphire tube . this is synthetic sapphire . the real sapphire , the gems , have impurities in them , of other metals , which give them the nice colors , particularly the blue . princess kate has a blue sapphire ring which belonged to her husband 's mother , princess di , before her . and so , these are very valuable ones . but synthetic sapphire is also expensive but not in the same class as a natural gem . brady : what can nature do that the guys at the sapphire factory ca n't do ? professor : nature has time . the people who grow this will take hours or days or perhaps weeks to grow it . nature can spend thousands or millions of years growing a particular gem and therefore they can heat it up and cool it down in natural surroundings , in volcanoes ... or whatever far more slowly than people can afford to do industrially . there 's a lot of argument whether you should call it aluminum or aluminium now , there is n't a totally correct one because both forms are acceptable . but , all or nearly all chemists use aluminium because it 's very important to use a standardized nomenclature right across the world . and i think aluminium sounds nicer . student : hi professor , my question is is it aluminum or aluminium ? 'cause i want to know what to call my aluminium model . apparantly , there was a decision in 1990 by iupac the international union of pure and applied chemistry that it should definately be called a l u m i n i u m but then they relented three years later and said you could use aluminum as well . but if you 're a serious chemist you really need to say aluminium , because otherwise people wo n't find your papers , your publications , when they search because they 'll almost certainly put an `` i '' ' in the name . aluminium is frequently used or used [ to be ] frequently used kkfor sauce pans , for cooking in because it 's easy to make , easy to machine and particularly when people used electric stoves it was easy to make a flat bottom so that you got good contact between the electric element and the sauce pan . the problem with aluminium sauce pans is that if you 're cooking some fairly acidic food , for example boiling lemons or rhubarb something like that which is quite acidic you can dissolve some of the aluminium and people got quite worried about getting aluminium in their food . also , if you cook red cabbage , which is an indicator ; blue for alkali , red for acid , then if you boil it in an aluminium sauce pan it goes blue . and earlier in my carreer i used a red cabbage together with a white one to make a union jack , a u.k. flag with a mixture of red and blue-red cabbage and the white from the white cabbage . unfortionately , i 've lost the photo ; brady is very cross with me . but it was quite fun cooking it in the kitchen . i did it once myself , but it was such a lot of work that the second time a got one of my students to do it .

voosh ! ] and the aluminium was completely dissolved up forming aluminium chloride and copper metal . in my own research , aluminium is quite important ; quite a lot of our equipment uses aluminium .

while demonstrating a reaction to his children , the professor accidentally dissolved aluminium cupcake containers with copper chloride . can you write the equation for this reaction ?

today we 're talking about weird materials that we use in space , in robots and in your mouth . i 'm talking about shape memory alloys . like the name says , these are metals that remember different shapes . to understand how these metals work , we 've got to talk about atoms and organizing . let 's talk about atoms first . atoms are tiny bits of matter that you can not see with your eye yet they make up everything in our world , from the chair that you 're sitting on to your cell phone . atoms have some surprising ways of behaving too . we 'll talk about that shortly . now , how big is an atom ? well , imagine pulling one of your hairs out of your head and whittling it like a stick 100,000 times . one of those shavings would be the width of an atom . they 're that small . now let 's talk about atoms and organizing . you may not know this , but atoms arrange themselves similar to the way we humans arrange ourselves . sometimes they sit in rows , like we do on a bus or an airplane . we call that seating arrangement a phase . other times they sit diagonal from each other , sort of like seats in a movie theater or sports stadium . this is another phase . when atoms move from one seating to another this is called a phase change . phase changes are all around us . you may already know about water 's phases : solid , liquid and gas . many other materials have phases like that too . some of them have several solid phases . ok. back to those shape memory alloys we mentioned before . when we say that the metals remember their different shapes , what we 're really saying is they remember different seating arrangements of atoms . when the atoms rearrange , the metal moves from one shape to another . let 's look at a phase change in action . here i have a metal wire that is made out of nickel and titanium . this metal wire is a shape memory alloy , and i 'm going to make it switch between its different shapes using heat from a lighter . watch this . i 'm going to wrap this wire around my finger and then heat it . amazing ! that wire returns to a straight line , when i heat it . let 's try that again . i 'm going to wrap it around my finger , and heat it . yep , that 's still amazing . not only is it amazing , this is weird , because metals generally do n't do that . here 's a paper clip . when i heat it , i get nothing . what we 're seeing is the shape memory wire changing phases when it gets hot . when the wire is cold , atoms are in a diagonal arrangement , like the movie theater seating , we talked about before . we call this a monoclinic arrangement , and scientists will call this phase martensite . when i heated up the wire , the atoms moved into columns like airplane seating . this is a cubic arrangement . scientists will call this phase austenite . so when we added the heat , the atoms shifted positions seamlessly , and they 'll do this forever . they have this coordinated motion , just like members of a tireless marching band . each makes a small shift , but all together those small shifts create a totally different pattern . so that 's pretty cool , but where do we use these materials ? well , if you look in the sky tonight , shape memory alloys are at work - on mars . they 're used to move panels on the mars rover , so that it can study the environment . like our metal straightened when it was heated , the metals holding the panels will move when electrically heated . when we stop heating the shape memory metal , the panel will return back , due to an opposing spring . back on earth , shape memory alloys are used to open up clogged arteries as stents , which are small collapsible springs that force open passages . shape memory alloys are also used to move robots , toy butterflies , teeth in braces , and for a perfect fit every time , shape memory wires are used as underwires in bras . now you know victoria 's secret . by popping a bra into the dryer , it 'll be brand new every time . so whether it 's on mars or in your mouth , small atomic movements can create huge changes , and understanding the way atoms behave allows us to make materials that make our world a better place .

we 'll talk about that shortly . now , how big is an atom ? well , imagine pulling one of your hairs out of your head and whittling it like a stick 100,000 times .

how big is an atom ?

cancer is like a car crash . your body typically regulates the speed at which your cells divide , but sometimes , cancer cuts the brake lines , and your cells divide too quickly , accumulating mutations that cause them to veer away from their original function , form dangerous tumors , and land you in the hospital . cancer is basically an inability of the body to control the speed at which cells divide . when cells divide too quickly , they can often accumulate mutations that cause them to ignore their original function in the body , forming tumors . in turn , these tumors may interfere with the natural processes of the body , such as digestion and respiration , potentially leading to death . typically , your body has a number of genetic mechanisms to control how fast your cells divide . one of these genes is brca1 , which stands for breast cancer susceptibility gene 1 . brca1 belongs to a class of genes called tumor suppressor genes . tumor suppressor genes are involved in regulating how fast a cell divides . normally , cell division follows an orderly process called the cell cycle , which is basically the life cycle of a cell . within the cell cycle is a series of checkpoints , where proteins , such as the one produced by brca1 , regulate how fast the cell may proceed . how does it do this ? brca1 helps repair some forms of mutation in your dna . if your dna is damaged , brca1 keeps the cell from dividing until the mutation is repaired . you have two copies of the brca1 gene in every cell of your body . one copy you inherited from mom , the other from dad . this redundancy is a good a thing because you only need one functioning brca1 gene in a cell to regulate the cell cycle . but it 's important to note that while these copies have a similar function they 're not necessarily the same . in fact , there are hundreds of variations , or alleles , of brca1 . some regulate the cell cycle more effectively than others . in other words , some people are born with better regulating and repair mechanisms than others . and in some cases , mutations may render brca1 ineffective . when this happens , cells with damaged dna are allowed to divide . as they divide , these cells may accumulate additional mutations . these mutations may cause the cell to become less specialized and stop performing its original function in the tissue . if this occurs , then there 's a greater chance they 'll develop into cancer cells . while we all have the gene , such as brca1 , that can cause cancer , it 's only when these genes fail at their function that problems develop . having an ineffective or mutated version of brca1 can increase your susceptibility to cancer , much like driving with bad brakes increases the risk of an accident .

brca1 belongs to a class of genes called tumor suppressor genes . tumor suppressor genes are involved in regulating how fast a cell divides . normally , cell division follows an orderly process called the cell cycle , which is basically the life cycle of a cell . within the cell cycle is a series of checkpoints , where proteins , such as the one produced by brca1 , regulate how fast the cell may proceed . how does it do this ?

what is the specific role of brca1 in the cell ?

to many , one of the coolest things about `` game of thrones '' is that the inhabitants of the dothraki sea have their own real language . and dothraki came hot on the heels of the real language that the na'vi speak in `` avatar , '' which , surely , the na'vi needed when the klingons in `` star trek '' have had their own whole language since 1979 . and let 's not forget the elvish languages in j.r.r . tolkien 's `` lord of the rings '' trilogy , especially since that was the official grandfather of the fantasy conlangs . `` conlang '' is short for `` constructed language . '' they 're more than codes like pig latin , and they 're not just collections of fabricated slang like the nadsat lingo that the teen hoodlums in `` a clockwork orange '' speak , where `` droog '' from russian happens to mean `` friend . '' what makes conlangs real languages is n't the number of words they have . it helps , of course , to have a lot of words . dothraki has thousands of words . na'vi started with 1,500 words . fans on websites have steadily created more . but we can see the difference between vocabulary alone and what makes a real language from a look at how tolkien put together grand old elvish , a conlang with several thousands words . after all , you could memorize 5,000 words of russian and still be barely able to construct a sentence . a four-year-old would talk rings around you . that 's because you have to know how to put the words together . that is , a real language has grammar . elvish does . in english , to make a verb past , you add an `` -ed . '' wash , washed . in elvish , `` wash '' is `` allu '' and `` washed '' is `` allune . '' real languages also change over time . there 's no such thing as a language that 's the same today as it was a thousand years ago . as people speak , they drift into new habits , shed old ones , make mistakes , and get creative . today , one says , `` give us today our daily bread . '' in old english , they said , `` urne gedaeghwamlican hlaf syle us todaeg . '' things change in conlangs , too . tolkien charted out ancient and newer versions of elvish . when the first elves awoke at cuiviénen , in their new language , the word for `` people '' was `` kwendi , '' but in the language of one of the groups that moved away , teleri , over time , `` kwendi '' became `` pendi , '' with the `` k '' turning into a `` p. '' and just like real languages , conlangs like elvish split off into many . when the romans transplanted latin across europe , french , spanish , and italian were born . when groups move to different places , over time , their ways of speaking grow apart , just like everything else about them . thus , latin 's word for hand was `` manus , '' but in french , it became `` main , '' while in spain it became `` mano . '' tolkien made sure elvish did the same kind of thing . while that original word `` kwendi '' became `` pendi '' among the teleri , among the avari , who spread throughout middle earth , it became `` kindi '' when the `` w '' dropped out . the elvish varieties tolkien fleshed out the most are quenya and sindarin , and their words are different in the same way french and spanish are . quenya has `` suc '' for `` drink , '' sindarin has `` sog . '' and as you know , real languages are messy . that 's because they change , and change has a way of working against order , just like in a living room or on a bookshelf . real languages are never perfectly logical . that 's why tolkien made sure that elvish had plenty of exceptions . lots of verbs are conjugated in ways you just have to know . take even the word `` know . '' in the past , it 's `` knew , '' which is n't explained by any of the rules in english . oh well . in elvish , `` know '' is `` ista , '' but `` knew '' is `` sinte . '' oh well . the truth is , though , that elvish is more a sketch for a real language than a whole one . for tolkien , elvish was a hobby rather than an attempt to create something people could actually speak . much of the elvish the characters in the `` lord of the rings '' movies speak has been made up since tolkien by dedicated fans of elvish based on guesses as to what tolkien would have constructed . that 's the best we can do for elvish because there are no actual elves around to speak it for us . but the modern conlangs go further . dothraki , na'vi , and klingon are developed enough that you can actually speak them . here 's a translation of `` hamlet '' into klingon , although performing it would mean getting used to pronouncing `` k '' with your uvula , that weird , cartoony thing hanging in the back of your throat . believe it or not , you actually do that in plenty of languages around the world , like eskimo ones . pronouncing elvish is much easier , though . so , let 's take our leave for now from this introduction to conlangs in elvish and the other three conlangs discussed with a heartfelt quad-conlangual valedictory : `` a na marie ! '' `` hajas ! '' na'vi 's `` kiyevame ! '' `` qapla ! '' and `` goodbye ! ''

tolkien charted out ancient and newer versions of elvish . when the first elves awoke at cuiviénen , in their new language , the word for `` people '' was `` kwendi , '' but in the language of one of the groups that moved away , teleri , over time , `` kwendi '' became `` pendi , '' with the `` k '' turning into a `` p. '' and just like real languages , conlangs like elvish split off into many . when the romans transplanted latin across europe , french , spanish , and italian were born .

the reason people is kwendi in cuivienen but pendi in teleri is

trillions of bacteria , viruses , and fungi live on or inside of us , and maintaining a good , balanced relationship with them is to our advantage . together , they form the gut microbiome , a rich ecosystem that performs a variety of functions in our bodies . the bacteria in our guts can break down food the body ca n't digest , produce important nutrients , regulate the immune system , and protect against harmful germs . we do n't yet have the blueprint for exactly which good bacteria a robust gut needs , but we do know that it 's important for a healthy microbiome to have a variety of bacterial species . many factors affect our microbiomes , including our environment , medications like antibiotics , and even whether we were delivered by c-section or not . diet , too , is emerging as one of the leading influences on the health of our guts . and while we ca n't control all these factors , we can manipulate the balance of our microbes by paying attention to what we eat . dietary fiber from foods like fruits , vegetables , nuts , legumes , and whole grains is the best fuel for gut bacteria . when bacteria digest fiber , they produce short chain fatty acids that nourish the gut barrier , improve immune function , and can help prevent inflammation , which reduces the risk of cancer . and the more fiber you ingest , the more fiber-digesting bacteria colonize your gut . in a recent study , scientists exchanged the regular high-fiber diets of a group of rural south africans with the high-fat , meat-heavy diets of a group of african-americans . after just two weeks on the high-fat , low-fiber , western-style diet , the rural african group showed increased inflammation of the colon , as well as a decrease of butyrate . that 's a short chain fatty acid thought to lower risk of colon cancer . meanwhile , the group that switched to a high-fiber , low-fat diet had the opposite result . so what goes wrong with our gut bacteria when we eat low-fiber processed foods ? lower fiber means less fuel for the gut bacteria , essentially starving them until they die off . this results in less diversity and hungry bacteria . in fact , some can even start to feed on the mucus lining . we also know that specific foods can affect gut bacteria . in one recent microbiome study , scientists found that fruits , vegetables , tea , coffee , red wine , and dark chocolate were correlated with increased bacterial diversity . these foods contain polyphenols , which are naturally occurring antioxidant compounds . on the other hand , foods high in dairy fat , like whole milk , and sugar-sweetened sodas were correlated with decreased diversity . how food is prepared also matters . minimally processed , fresh foods generally have more fiber and provide better fuel . so lightly steamed , sautéed , or raw vegetables are typically more beneficial than fried dishes . there are also ways of preparing food that can actually introduce good bacteria , also known as probiotics , into your gut . fermented foods are teeming with helpful probiotic bacteria , like lactobacillus and bifidobacteria . originally used as a way of preserving foods before the invention of refrigeration , fermentation remains a traditional practice all over the world . foods like kimchi , sauerkraut , tempeh , and kombucha provide variety and vitality to our diets . yogurt is another fermented food that can introduce helpful bacteria into our guts . that does n't necessarily mean that all yogurt is good for us , though . brands with too much sugar and not enough bacteria may not actually help . these are just general guidelines . more research is needed before we fully understand exactly how any of these foods interact with our microbiomes . we see positive correlations , but the insides of our guts are difficult places to make direct observations . for instance , we do n't currently know whether these foods are directly responsible for the changes in diversity , or if something more complicated is happening . while we 're only beginning to explore the vast wilderness inside our guts , we already have a glimpse of how crucial our microbiomes are for digestive health . the great news is we have the power to fire up the bacteria in our bellies . fill up on fibers , fresh and fermented foods , and you can trust your gut to keep you going strong .

meanwhile , the group that switched to a high-fiber , low-fat diet had the opposite result . so what goes wrong with our gut bacteria when we eat low-fiber processed foods ? lower fiber means less fuel for the gut bacteria , essentially starving them until they die off . this results in less diversity and hungry bacteria .

all of the following vegetable preparation methods help provide more fuel for gut bacteria except :

what do an ancient greek philosopher and a 19th century quaker have in common with nobel prize-winning scientists ? although they are separated over 2,400 years of history , each of them contributed to answering the eternal question : what is stuff made of ? it was around 440 bce that democritus first proposed that everything in the world was made up of tiny particles surrounded by empty space . and he even speculated that they vary in size and shape depending on the substance they compose . he called these particles `` atomos , '' greek for indivisible . his ideas were opposed by the more popular philosophers of his day . aristotle , for instance , disagreed completely , stating instead that matter was made of four elements : earth , wind , water and fire , and most later scientists followed suit . atoms would remain all but forgotten until 1808 , when a quaker teacher named john dalton sought to challenge aristotelian theory . whereas democritus 's atomism had been purely theoretical , dalton showed that common substances always broke down into the same elements in the same proportions . he concluded that the various compounds were combinations of atoms of different elements , each of a particular size and mass that could neither be created nor destroyed . though he received many honors for his work , as a quaker , dalton lived modestly until the end of his days . atomic theory was now accepted by the scientific community , but the next major advancement would not come until nearly a century later with the physicist j.j. thompson 's 1897 discovery of the electron . in what we might call the chocolate chip cookie model of the atom , he showed atoms as uniformly packed spheres of positive matter filled with negatively charged electrons . thompson won a nobel prize in 1906 for his electron discovery , but his model of the atom did n't stick around long . this was because he happened to have some pretty smart students , including a certain ernest rutherford , who would become known as the father of the nuclear age . while studying the effects of x-rays on gases , rutherford decided to investigate atoms more closely by shooting small , positively charged alpha particles at a sheet of gold foil . under thompson 's model , the atom 's thinly dispersed positive charge would not be enough to deflect the particles in any one place . the effect would have been like a bunch of tennis balls punching through a thin paper screen . but while most of the particles did pass through , some bounced right back , suggesting that the foil was more like a thick net with a very large mesh . rutherford concluded that atoms consisted largely of empty space with just a few electrons , while most of the mass was concentrated in the center , which he termed the nucleus . the alpha particles passed through the gaps but bounced back from the dense , positively charged nucleus . but the atomic theory was n't complete just yet . in 1913 , another of thompson 's students by the name of niels bohr expanded on rutherford 's nuclear model . drawing on earlier work by max planck and albert einstein he stipulated that electrons orbit the nucleus at fixed energies and distances , able to jump from one level to another , but not to exist in the space between . bohr 's planetary model took center stage , but soon , it too encountered some complications . experiments had shown that rather than simply being discrete particles , electrons simultaneously behaved like waves , not being confined to a particular point in space . and in formulating his famous uncertainty principle , werner heisenberg showed it was impossible to determine both the exact position and speed of electrons as they moved around an atom . the idea that electrons can not be pinpointed but exist within a range of possible locations gave rise to the current quantum model of the atom , a fascinating theory with a whole new set of complexities whose implications have yet to be fully grasped . even though our understanding of atoms keeps changing , the basic fact of atoms remains , so let 's celebrate the triumph of atomic theory with some fireworks . as electrons circling an atom shift between energy levels , they absorb or release energy in the form of specific wavelengths of light , resulting in all the marvelous colors we see . and we can imagine democritus watching from somewhere , satisfied that over two millennia later , he turned out to have been right all along .

though he received many honors for his work , as a quaker , dalton lived modestly until the end of his days . atomic theory was now accepted by the scientific community , but the next major advancement would not come until nearly a century later with the physicist j.j. thompson 's 1897 discovery of the electron . in what we might call the chocolate chip cookie model of the atom , he showed atoms as uniformly packed spheres of positive matter filled with negatively charged electrons .

j.j. thomson discovered which atomic particle ?

why are most manhole covers round ? sure , it makes them easy to roll and slide into place in any alignment but there 's another more compelling reason involving a peculiar geometric property of circles and other shapes . imagine a square separating two parallel lines . as it rotates , the lines first push apart , then come back together . but try this with a circle and the lines stay exactly the same distance apart , the diameter of the circle . this makes the circle unlike the square , a mathematical shape called a curve of constant width . another shape with this property is the reuleaux triangle . to create one , start with an equilateral triangle , then make one of the vertices the center of a circle that touches the other two . draw two more circles in the same way , centered on the other two vertices , and there it is , in the space where they all overlap . because reuleaux triangles can rotate between parallel lines without changing their distance , they can work as wheels , provided a little creative engineering . and if you rotate one while rolling its midpoint in a nearly circular path , its perimeter traces out a square with rounded corners , allowing triangular drill bits to carve out square holes . any polygon with an odd number of sides can be used to generate a curve of constant width using the same method we applied earlier , though there are many others that are n't made in this way . for example , if you roll any curve of constant width around another , you 'll make a third one . this collection of pointy curves fascinates mathematicians . they 've given us barbier 's theorem , which says that the perimeter of any curve of constant width , not just a circle , equals pi times the diameter . another theorem tells us that if you had a bunch of curves of constant width with the same width , they would all have the same perimeter , but the reuleaux triangle would have the smallest area . the circle , which is effectively a reuleaux polygon with an infinite number of sides , has the largest . in three dimensions , we can make surfaces of constant width , like the reuleaux tetrahedron , formed by taking a tetrahedron , expanding a sphere from each vertex until it touches the opposite vertices , and throwing everything away except the region where they overlap . surfaces of constant width maintain a constant distance between two parallel planes . so you could throw a bunch of reuleaux tetrahedra on the floor , and slide a board across them as smoothly as if they were marbles . now back to manhole covers . a square manhole cover 's short edge could line up with the wider part of the hole and fall right in . but a curve of constant width wo n't fall in any orientation . usually they 're circular , but keep your eyes open , and you just might come across a reuleaux triangle manhole .

a square manhole cover 's short edge could line up with the wider part of the hole and fall right in . but a curve of constant width wo n't fall in any orientation . usually they 're circular , but keep your eyes open , and you just might come across a reuleaux triangle manhole .

which of the following is a curve of constant width ?

take a moment to read the following . how was that ? frustrating ? slow ? what were those sentences about ? they 're actually a simulation of the experience of dyslexia , designed to make you decode each word . those with dyslexia experience that laborious pace every time they read . when most people think of dyslexia , they think of seeing letters and words backwards , like seeing `` b '' as `` d '' and vice versa , or they might think people with dyslexia see `` saw '' as `` was '' . the truth is people with dyslexia see things the same way as everyone else . dyslexia is caused by a phonological processing problem , meaning people affected by it have trouble not with seeing language but with manipulating it . for example , if you heard the word cat and then someone asked you , `` remove the 'c ' , '' what word would you have left ? at . this can be difficult for those with dyslexia . given a word in isolation , like fantastic , students with dyslexia need to break the word into parts to read it : fan , tas , tic . time spent decoding makes it hard to keep up with peers and gain sufficient comprehension . spelling words phonetically , like s-t-i-k for stick and f-r-e-n-s for friends is also common . these difficulties are more widespread and varied than commonly imagined . dyslexia affects up to one in five people . it occurs on a continuum . one person might have mild dyslexia while the next person has a profound case of it . dyslexia also runs in families . it 's common to see one family member who has trouble spelling while another family member has severe difficulty decoding even one syllable words , like catch . the continuum and distribution of dyslexia suggests a broader principle to bear in mind as we look at how the brains of those with dyslexia process language . neurodiversity is the idea that because all our brains show differences in structure and function , we should n't be so quick to label every deviation from `` the norm '' as a pathological disorder or dismiss people living with these variations as `` defective . '' people with neurobiological variations like dyslexia , including such creative and inventive individuals as picasso , muhammad ali , whoopi goldberg , steven spielberg , and cher , clearly have every capacity to be brilliant and successful in life . so , here 's the special way the brains of those with dyslexia work . the brain is divided into two hemispheres . the left hemisphere is generally in charge of language and , ultimately , reading , while the right typically handles spatial activities . fmri studies have found that the brains of those with dyslexia rely more on the right hemisphere and frontal lobe than the brains of those without it . this means , when they read a word , it takes a longer trip through their brain and can get delayed in the frontal lobe . because of this neurobiological glitch , they read with more difficulty . but those with dyslexia can physically change their brain and improve their reading with an intensive , multi-sensory intervention that breaks the language down and teaches the reader to decode based on syllable types and spelling rules . the brains of those with dyslexia begin using the left hemisphere more efficiently while reading , and their reading improves . the intervention works because it locates dyslexia appropriately as a functional variation in the brain , which , naturally , shows all sorts of variations from one person to another . neurodiversity emphasizes this spectrum of brain function in all humans and suggests that to better understand the perspectives of those around us , we should try not only to see the world through their eyes but understand it through their brains .

the brain is divided into two hemispheres . the left hemisphere is generally in charge of language and , ultimately , reading , while the right typically handles spatial activities . fmri studies have found that the brains of those with dyslexia rely more on the right hemisphere and frontal lobe than the brains of those without it .

which hemisphere is responsible for language learning ?

cities are a big deal : we pretty much all have to live in them ; we should try hard to get them right . so few cities are nice ; very , very few out of many thousands are really beautiful . embarrassingly , the more appealing ones tend to be old , which is weird because we 're mostly much better at making things now : cars , planes , or phones . why not , then , cities ? it 's crazy to settle for this and to leave something so important to chance . we need to get more scientific and identify the principles that determine how a city gets to be pretty or ugly . it 's not a mystery why we like some cities so much better than others . this is a manifesto about how to make attractive cities . there are six fundamental things a city needs to get right . 1 . not too chaotic ; not too ordered one of the things we really love in cities is order . order means balance , symmetry and repetition ; it means the same thing happening again and again , and the left side matching the right side . order is one of the reasons so many people love paris . but most cities are a complete mess . when it 's a mess , it seems like no one is in charge . and that 's worrying . it 's horrible when everything is jumbled up . a pitched roof next to a flat roof , a stark geometrical box next to a muddled car park , high rise towers that look as if they 've been placed at random , like teeth in a gaping mouth . we generally have an itch to straighten things out , and when we ca n't , it 's frustrating . the same urge is there when we look at cities . often , it 's not skyscrapers that we mind in the city , it 's skyscrapers that have been dumped without planning , like they are increasingly in london , whereas new york or chicago shows the ordered way that we love . however , you have to keep something else in mind : excessive order can be just as much of a problem . too much regularity can be soul destroying . too much order feels rigid and alien . it can be bleak , relentless , and harsh . so the ideal we 're seeking is variety and order . this is the idea in a square in telč in the czech republic : where every house is the same width and height but within that ordered pattern , every house has been allowed freedom at the level of form and colour or in java-eiland in amsterdam where the pattern is quite strict : each house has the same height and width , the color range is restricted , but within this grid , each unit is completely individual . we 're perfectly in the middle between chaos and boringness here . and that 's what humans adore . that 's what more and more cities should have : order and variety . so as a general rule : too much mess , and it 's off putting , but too much simple order , and it 's boring . what we crave it 's organized complexity which you can see as much here : as here : now , for the second thing that makes cities beautiful : they have to have visible life . there are streets that are dead and streets that are alive and in general , we crave the live ones . this is a live street in hong kong . this is a live scene in venice . in the 18th century , the painter canaletto specialized in pictures of cities everyone loved because they 're full of life . there 's always plenty going on . in this painting we can see a stonemason 's yard . the work sheds are rough , but they 're charming . it 's fascinating to see what people are up to . how do they load those huge blocks onto the gondolas ? the life of the city is on display , and we 're primed to love this . contrast this with dead streets of many modern cities . today , the places where a lot of the work gets done look dull and dead . they 're spaced out along huge highways , and you never go there unless you happen to work there yourself because there 's nothing to see . and most office buildings are brutally anonymous ; the people inside might be working in all sorts of fascinating stuff , but we just do n't know , and it 's disorienting and cold . the street levels are dead . contrast this with the streets we all love , where you can see things going on : a bakery , a cobbler 's shop , and markets selling carpets , a burger bar , a bookshop ; these are streets we love because they 're full of life . more and more , in modern cities , we 've hidden life away . we have lots of dead sheds , and dead towers , connected up by dead motorways where you can barely glimpse your fellow humans . rather than the old alleyways where you can see people at work , look them in the eye as they walk down the road and feel connected to others . modern planners have become obessed by hiding technology rather than trying to make it nice to look at . today we'de be outraged if we heard a huge pipeline was gon na be slapped across a lovely river ; we 'd be up in arms ! but we book trips to go and see the roman pont du gard in southern france . that 's because it 's built for beauty and practicality . we think it 's the pipe we hate . it 's not . it 's just the ugliness . so let 's make sure our streets are full of life , full of people doing stuff you can see through the windows . that 's what make certain cities so attractive to walk along : the work is on show , the people are proud of what they 're doing and happy to let the world notice and appreciate the practical side of things . there 's a third principle of good cities : they are compact . in the past , being able to be alone or just with your partner or family , was at first , a huge achievement . only the largest class , the poor , lived huddled together and it was horrid . as soon as people had money , they wanted to move out , and have their own plots . through the later decades of 20th century , more and more people tucked themselves away in a private realm . and it 's been a disaster . it 's become deadly , cold , and boring , and very , very wasteful on the environment . a compact city like barcelona swallows a fraction of the energy of a sprawling one like phoenix , in arizona . we 've built a world of endless dead dormitory suburbs connected by sterile wide motorways all because we labor under the false impression that we want to be far away from other people . but in fact it 's wonderful to have the balancing moderating influence of living close to other people in uplifting surroundings . that 's why we need tightly packed , well-ordered cities with lots of squares in public places in which we can hang out . all the most beautiful compact cities have squares . yet , the art of the square has gone into terrible decline . we keep promoting the invention of mobile phones , but no one 's built a good square anywhere on this planet for decades . it 's not rocket science though . look at the piazza di santa maria in trastevere , rome . it 's a public place , but intimate and closed enough to feel like an extension of your home . lounging about here , having a coffee or a beer , reading a paper , you get to be around other people , their moderating , cheering affect is restoring . it takes you away from the over intense , couple obsessed atmosphere of the home . there 's an art to a good square : it should be neither too big nor too small , anything over 30 meters in diameter starts become too large by which we mean : the individual become overly small relative to the space around them , creating a sense of alienation and dislocation . in a good square you should be able to see the face of a person across the square , you could if need be hail someone walking on the other side . the ideal square must offer a feeling of containment , but not claustrophobia . there 's another principle of good cities to do with orientation and mystery . by definition , cities are huge , but the cities that a lot of people love also have lots of little back streets and small lanes where you can feel cozy and get a bit lost . we 're drawn to the sense of mystery and enclosure that these streets offer . it 's actually lovely to get a bit lost . a warren of alleyways can feel homely and intimate . at cartagena , in colombia , the balconies nearly touch across the street- you can see your neighbors having breakfast , you know when they 've gone to bed , what time the children do their homework on a sunday evening . the fact that everyone is little bit on display a lot of the time tends to make people nicer . they do n't shout at each other quite so much . they put flowers on the table more often . we like it , but we forget that we do , and we do n't quite know how to ask for it . modern planners and developers give us maximum privacy because they suppose that 's what we all want , and because they insist that cars and lorries , which like a lot more space than people , are the most important things in the world . of course we need balance between small streets and big ones . necessarily , cities are large . we love small streets , but they 're a nightmare when you have to go any distance . so the ideal is to have big boulevards , grand , wide straight places , and also little warrens of streets . we need cities that offer us two important pleasures : the pleasures of mystery and the pleasures of orientation . let 's think about scale now . modern cities are all about big things . joseph campbell once wrote : `` if you want to see what a society really believes in , look at what the biggest buildings on the horizon are dedicated to '' . the biggest most prominent things tell us about the actual , rather than admitted priorities of a society . we do n't collectively say we worship sports shoe corporations , tax specialists , the oil industry , and pharmaceuticals . our cities , however , tell another story . they 're full of enormous towers devoted to just these things . that is a bit depressing . as humans , we do n't mind things being big , per se , we do n't mind being humbled , so long as the things we are bowing to deserve homage , like a beautiful mosque , or a cathedral , or a museum . but we 've allowed our cities to be hijacked by aggressive commercial interests , by towers that honor not god , or love , or humanity , but pizza corporations and hedge funds . they exist because we 've made a big dumb collective mistake : we focused on who owns land , but we do n't think about who owns space , who has air rights . and in the end , who has air rights determines what you can see from your window . we suggest that the ideal height for any city block is five stories high . no more . above that people start to feel small , insignificant , and trivial . so we say : cut down those towers and pack everything into five stories . make it dense , compact , and tight , like they do in some parts of berlin , amsterdam , london , and paris , the bits we love . of course , occasionally there can be a huge building , but let 's keep it for something really special , something all of humanity can love . towers have to be worthy of their prominence , they must be aligned with our best ambitions and long-term needs . finally , make it local . somethings should be the same everywhere . we do n't expect there to be a uniquely venezuelan telephone or a distinctively icelandic bicycle . but , we do n't want buildings to look the same everywhere . it 's hugely disappointing when you fly somewhere for hours , land , and feel you could be anywhere . the problem is n't just that we like a bit of variation for it 's own sake ; because of climate , history and social traditions , each society really does have different needs , different strengths and weaknesses . there are many distinct styles of happiness ; many good and varied ways of conductive and collective life . the sameness of cities is a problem because it reveals how far each of us must be from engaging with an specific character of it 's own place . it 's like wearing the same clothes in all climates , or speaking exactly the same way irrespective of who you 're talking to . cities need to have strong characters connected to the use of distinctive local materials and forms . the pale sandstone , of millbrae crescent in glasgow 's south side , is a local material , a medium grained , carboniferous , blond sandstone- formed when the scottish landmass lay near the equator . or around cambridge , brick from the local yellowish gault clay is a major traditional material . or think of the way the great australian architect , glenn murcutt , found ways to put up buildings that reflected the distinctive character australian life . so the law should be : do n't make your city from buildings that could be just anywhere ; find a style of architecture that reflects what makes your location specific . the obstacles to building beautiful cities and not economic . collectively , we 've got enough money . we face two main problems : firstly , an intellectual confusion around beauty , and secondly , lack of political will . the intellectual confusion is : we think no one has a right to say what 's beautiful and what 's ugly ; we get worried about who decides ; we think beauty is subjective , so surely no one should say anything about it . it 's a very understandable qualm , but it 's horribly useful to greedy property developers . it 's such a relief to these people to learn that there is no such thing as beauty ; it means they can get away with murder . we may not agree to the very last point about what a beautiful city is , but we know an ugly one when we see it . no one 's ever willingly taken holiday in frankfurt-on-main or birmingham , and there are good reasons for this to do with an objective sense of beauty . so let 's stop being dangerously relativistic about this . yes ! there is such a thing as beauty . sydney and san francisco and bath and bordeaux have it and most other places do n't . the proof lies in the tourist statistics . let 's not just say that beauty is in the eye of the beholder ; that 's just a gift to the next wealthy idiot who wants to put up a horrible tower . the other obstacle is a lack of political will . we 've abandoned the design of cities to the greedy rich . we 've given up believing in democracy . we 've faced and have lost the battle between the public good and commercial opportunism . there will always be a greedy , slick lobby fighting for ugly development , but we can say no . beautiful cities have only ever been created when governments impose strict and ambitious regulations to keep the greedy , private guys in check . think of edinburgh 's amazing new town , which only got of the ground because the government established clear rules to keep developers in check . they were precise legislation detailing heights for buildings , quality of finish , the width of pavements , and a character of the skyline . that 's the only way you get beauty . they did n't leave it to the free market . do that , and you will have chaos . when governments give up on beauty , people start to hate all building . we become collectively despondent . we think we hate all building , and that we ca n't create beautiful places we get obsessed by restorations and opposed to anything new , which is wrong because we need places to live . humanity has n't put up a single beautiful city since about 1905 . when venice was built , no one regretted the lagoons that had been swallowed up . the goal of building , should be to put up things that do n't leave us regretting the nature that 's been lost because the architecture is every bit the equal of the designs of nature . we can create more beautiful cities , but we have to confront opportunistic developers and our own intellectual confusions . governments can only create beauty if they have enough public backing . political will is ultimately about what all of us , the electorate , are asking for . that 's why we made this film and hope to awaken you to your power as citizens to help legislate for beautiful cities in the future these are the six rules . now , it 's time to fight to put them in action .

there 's an art to a good square : it should be neither too big nor too small , anything over 30 meters in diameter starts become too large by which we mean : the individual become overly small relative to the space around them , creating a sense of alienation and dislocation . in a good square you should be able to see the face of a person across the square , you could if need be hail someone walking on the other side . the ideal square must offer a feeling of containment , but not claustrophobia . there 's another principle of good cities to do with orientation and mystery . by definition , cities are huge , but the cities that a lot of people love also have lots of little back streets and small lanes where you can feel cozy and get a bit lost .

the video recommended that a good city should have a square . what was the approximate diameter for a good square ?

charles osborne began to hiccup in 1922 after a hog fell on top of him . he was n't cured until 68 years later and is now listed by guinness as the world record holder for hiccup longevity . meanwhile , florida teen jennifer mee may hold the record for the most frequent hiccups , 50 times per minute for more than four weeks in 2007 . so what causes hiccups ? doctors point out that a round of hiccups often follows from stimuli that stretch the stomach , like swallowing air or too rapid eating or drinking . others associate hiccups with intense emotions or a response to them : laughing , sobbing , anxiety , and excitement . let 's look at what happens when we hiccup . it begins with an involuntary spasm or sudden contraction of the diaphragm , the large dome-shaped muscle below our lungs that we use to inhale air . this is followed almost immediately by the sudden closure of the vocal chords and the opening between them , which is called the glottis . the movement of the diaphragm initiates a sudden intake of air , but the closure of the vocal chords stops it from entering the wind pipe and reaching the lungs . it also creates the characteristic sound : `` hic . '' to date , there is no known function for hiccups . they do n't seem to provide any medical or physiological advantage . why begin to inhale air only to suddenly stop it from actually entering the lungs ? anatomical structures , or physiological mechanisms , with no apparent purpose present challenges to evolutionary biologists . do such structures serve some hidden function that has n't yet been discovered ? or are they relics of our evolutionary past , having once served some important purpose only to persist into the present as vestigial remnants ? one idea is that hiccups began many millions of years before the appearance of humans . the lung is thought to have evolved as a structure to allow early fish , many of which lived in warm , stagnant water with little oxygen , to take advantage of the abundant oxygen in the air overhead . when descendants of these animals later moved onto land , they moved from gill-based ventilation to air-breathing with lungs . that 's similar to the much more rapid changes faced by frogs today as they transition from tadpoles with gills to adults with lungs . this hypothesis suggests that the hiccup is a relic of the ancient transition from water to land . an inhalation that could move water over gills followed by a rapid closure of the glottis preventing water from entering the lungs . that 's supported by evidence which suggests that the neural patterning involved in generating a hiccup is almost identical to that responsible for respiration in amphibians . another group of scientists believe that the reflex is retained in us today because it actually provides an important advantage . they point out that true hiccups are found only in mammals and that they 're not retained in birds , lizards , turtles , or any other exclusively air-breathing animals . further , hiccups appear in human babies long before birth and are far more common in infants that adults . their explanation for this involves the uniquely mammalian activity of nursing . the ancient hiccup reflex may have been adapted by mammals to help remove air from the stomach as a sort of glorified burp . the sudden expansion of the diaphragm would raise air from the stomach , while a closure of the glottis would prevent milk from entering the lungs . sometimes , a bout of hiccups will go on and on , and we try home remedies : sipping continuously from a glass of cold water , holding one 's breath , a mouthful of honey or peanut butter , breathing into a paper bag , or being suddenly frightened . unfortunately , scientists have yet to verify that any one cure works better or more consistently than others . however , we do know one thing that definitely does n't work .

it also creates the characteristic sound : `` hic . '' to date , there is no known function for hiccups . they do n't seem to provide any medical or physiological advantage .

the principal function of the diaphragm is :

hi there , i ’ m john green , this is crash course : world history and today we ’ re going to talk about islam , which like christianity and judaism grew up on the east coast of the mediterranean but unlike christianity and judaism is not terribly well understood in the west . for instance , you probably know what this is and what this is , you probably don ’ t know what that is . google it . mr. green mr. green why do you think people know so little about islamic history ? did you just ask an interesting non-annoying question , me from the past ? i think we don ’ t know about early islamic history because we don ’ t learn about it , me from the past , because we don ’ t learn about it , because we ’ re taught that our history is the story of christianity in europe , when in fact our history is the story of people on the planet , so let ’ s try to learn something today . [ theme music ] so in less than 200 years islam went from not existing to being the religious and political organizing principal of one of the largest empires in the world . and that story begins in the 7th century ce when the angel gabriel appeared to muhammad , a 40-ish guy who made his living as a caravan trader and told him to begin reciting the word of god . initially , this freaked muhammad out , as , you know , it would—but then his wife and a couple of other people encouraged him and slowly he came to accept the mantle as prophet . a few things to know about the world islam entered : first , muhammad ’ s society was intensely tribal . he was a member of the quraysh tribe , living in mecca and tribal ties were extremely important . also , at the time , the arabian peninsula was like this crazy religious melting pot . like most tribal arabs worshipped gods very similar to the mesopotamian gods you ’ ll remember from episode 3 . and by the time of muhammad , cult statutes of many of those gods had been collected in his hometown of mecca in this temple-like structure called the kaaba . but arabia was also a home for monotheisms like christianity and judaism , even a bit of zoroastrianism . so the message that there was only god wouldn ’ t have been like as surprising to muhammad as it was , for instance , to abraham . also , and this will become very important , the northern part of arabia was sandwiched between the byzantine empire and the persian sassanian empire—and you ’ ll remember , those guys were always fighting . they were like snowboarders and skiers , or like the westboro baptist church and everyone else . at its core , islam is what we call a radical reforming religion—just like jesus and moses sought to restore abrahamic monotheism after what they perceived as straying , so too did muhammad . muslims believe that god sent muhammad as the final prophet to bring people back to the one true religion , which involves the worship of , and submission to , a single and all-powerful god . the quran also acknowledges abraham and moses and jesus among others as prophets , but it ’ s very different from the hebrew and christian bibles : for one thing it ’ s much less narrative , but also its the written record of the revelations muhammad received—which means its not written from the point of view of people , it is seen as the actual word of god . the quran is a really broad-ranging text , but it returns again and again to a couple themes . one is strict monotheism and the other is the importance of taking care of those less fortunate than you . the quran , says of the good person spends his substance—however much he himself may cherish it—upon his near of kin , and the orphans , and the needy , and the wayfarer , and the beggars , and for the freeing of human beings from bondage . these revelations also radically increased the rights of women and orphans , which was one of the reasons why mohammad ’ s tribal leaders weren ’ t that psyched about them . to talk more about islamic faith and practice , let ’ s go to the thought bubble . the five pillars of islam are the basic acts considered obligatory , at least by sunni muslims . first is the shahada or the profession of the faith : there is no god but god and muhammad is god ’ s prophet , which is sometimes translated as “ there is no god but allah and muhammad is allah ’ s prophet ” , which tries to make muslims sound other and ignores the fact that the arabic word for god—whether you are christian or jewish or muslim—is allah . second , salat , or ritual prayer five times a day—at dawn , noon , afternoon , sunset , and late evening—which are obligatory unless you haven ’ t hit puberty , are too sick , or are menstruating . keep it pg , thought bubble . third , sawm , the month-long fast during the month of ramadan , in which muslims do not eat or drink or smoke cigarettes during daylight hours . since ramadan is a lunar-calendar month , it moves around the seasons , and obviously it ’ s most fun during the winter , when days are shorter , and least fun during the summer , when days are both long and hot . fourth is zakat , or almsgiving , in which non-poor muslims are required to give a percentage of their income to the poor , and lastly hajj , the pilgrimage to mecca that muslims must try to fulfill at least once in their lives , provided they are healthy and have enough money . and there ’ s also more to understanding islam than just knowing the quran . like judaism with its talmud , and christianity with its lives of saints and writings of church fathers , islam has supplementary sacred texts , chief among which is the hadith , a collection of sayings and stories about the prophet . thanks thought bubble . oh , it ’ s time for the open letter ? magic . an open letter to the 72 virgins . oh , but first let ’ s check what ’ s in the secret compartment . huh , it ’ s andre the giant . did you know that andre the giant died a virgin- is a fact that i made up ? dear 72 virgins , hey there , it ’ s me , john green . did you know that not all hadiths were created equal ? some sayings of the prophet are really well sourced . like for instance , a good friend or a relative heard the prophet say something and then it ended up as a hadith . but some hadiths are terribly sourced like , not to be irreverent , but some of it is like middle school gossip ; like rachel told rebekah that her sister ’ s brother ’ s friend kissed justin bieber on the face . and the vast majority of muslims don ’ t treat terribly sourced hadiths as scripture . and the idea that you go to heaven and get 72 virgins is not in the quran ; it ’ s in a terribly sourced hadith so it is my great regret to inform you , 72 virgins , that in the eyes of almost all muslims you do not exist . best wishes , john green one more thing about islam : like christianity and judaism , it has a body of law . you might have heard of it - it ’ s called sharia . although we tend to think of sharia as this single set of laws that all muslims follow , that ’ s ridiculous ; there are numerous competing interpretations of sharia , just as there are within any legal tradition . so people who embraced this worldview were called muslims , because they submitted to the will of god , and they became part of the umma , or community of believers . this would be a good moment for an uma thurman joke , but sadly she is no longer famous . i ’ m sorry if you ’ re watching this , uma thurman . being part of the umma trumped all other ties , including tribal ties , which got muhammad into some trouble and brings us , at last , back to history . so as muhammad ’ s following in mecca grew , the umma aroused the suspicion of the most powerful tribe , the quraysh . and it didn ’ t matter that muhammad himself was born into the quraysh tribe because he wouldn ’ t shut up about how there was only one god , which was really bad news to the quraysh tribe because they managed the pilgrimage trade in mecca , and if all those gods were false , it would be a disaster economically . —although come to think of it , in the end the meccan pilgrimage business turned out just fine . so the quraysh forced muhammad and his followers out of mecca in 622 ce , and they headed to yithrab , also known as medina . this journey , also know as the hijra , is so important that it marks year 0 in the islamic calendar . in medina , muhammad severed the religion ’ s ties to judaism , turning the focus of prayer away from jerusalem to mecca . also in medina , the islamic community started to look a lot more like a small empire than like a church . like , jesus never had a country to run . but muhammad did almost from the beginning . and in addition to being an important prophet , he was a good general and in 630 , the islamic community took back mecca . they destroyed the idols in the kabaa , and soon islam was as powerful a political force in the region as it was a religious one . and it ’ s because the political and religious coexisted from the beginning , that there ’ s no separate tradition of civic and religious law like there is in christianity and judaism . very different from judaism and even from christianity—which you ’ ll remember debated very different from judaism and even from christianity—which you ’ ll remember debated for generations whether to be inclusive . —and more importantly than separating islam from other monotheisms , that really separated islam from the tribalism in arabia . so then when muhammad died in 632 ce , there wasn ’ t a religious vacuum left behind : muhammad was the final prophet , the revelation of the quran would continue to guide the umma throughout their lives . but the community did need a political leader , a caliph . and the first caliph was abu bakr , muhammad ’ s father-in-law , who was not without his opponents : many people wanted ali , muhammad ’ s son-in-law , to lead the community . and although he did become the fourth caliph , that initial disagreement — to radically oversimplify because we only have ten minutes — began the divide between the two of the major sects of islam : suuni and shi ’ a . and even today , sunnis muslims believe abu bakr was rightly elected the first caliph and shi ’ a muslims believe it should ’ ve been ali . to sunnis , the first four caliphs—abu bakr , umar , uthman , and ali— are known as the rightly guided caliphs , and many of the conservative movements in the islamic world today are all about trying to restore the islamic world to those glory days , which—like most glory days—were not unambiguously glorious . abu bakr stabilized the community after muhammad ’ s death , and began the process of recording the quran in writing , and started the military campaigns against the byzantine and sassanian empires that within 116 years would allow the islamic empire to go from this to this . his successor umar was both an uncommonly good general and a brilliant administrator but like so many other great men , he proved terrible at avoiding assassination . which led to the caliphate of uthman , who standardized the quran and continued both his predecessor ’ s tradition of conquest and his predecessor ’ s tradition of getting assassinated . then ali finally got his turn at caliph , but his ascension was very controversial , and it ultimately led to a civil war . which eventually led to the emergence of uthman ’ s tribe , the umayyads , as the dynasty ruling over an ever-expanding islamic empire for more than a hundred years . it ’ s common to hear that in these early years islam quote spread by the sword , and that ’ s partly true , unless you are — wait for it — the mongols . actually , as usual , the truth is more complicated : many people , including the mongols but also including lots of people in central and east asia , embraced islam without any military campaigns . and in fact , the quran says that religion must not be an act of compulsion , but this much is true : the early islamic empire was really good at winning wars . and situated as they were between two very wealthy empires—the byzantines and the sassanians—there was plenty to fight for . first to fall was the sassanians , the last non-muslim successor to the persian empire . they were relatively easy pickings because they ’ d been fighting the byzantines for like 300 years and were super tired . also they ’ d been recently struck by plague . plague , man , i ’ m telling you ; it ’ s like the red tortoise shell of history . but in those early days they did pry away some valuable territory like egypt and the holy land and eventually they got into spain . where various muslim dynasties would entrench themselves until being expelled in 1492 . but as a good as they were at making war , it ’ s still tempting to chalk up the arabs ’ success to , you know , the will of god . and certainly a lot of the people they conquered felt that way . wars in this part of the world didn ’ t just pit people against each other , they also pitted their gods against each other . so while the islamic empire didn ’ t require its subjects to convert to islam , their stunning successes certainly convinced a lot of people that this monotheism thing was legit . once again , john green proving super hip to the slang of today ’ s young ’ ns . also , you paid lower taxes if you converted , and just as taxes on cigarettes lead to people not wanting to smoke , taxes on worshiping your idols lead to people not wanting to worship them anymore . so in a period of time that was , historically speaking , both remarkably recent and remarkably short , a small group of people from an area of the world with no natural resources managed to create one of the great empires of the world and also one of its great religions . and that very fact may be why people of western european descent remain largely ignorant about this period . not only were the muslims great conquerors , they spawned an explosion of trade and learning that lasted hundreds of years . they saved many of the classical texts that form the basis of the “ western canon ” while europe was ignoring them and they paved the way for the renaissance . while it ’ s important to remember that much of the world between spain and the indus river wasn ’ t arabized , most of it was so thoroughly islamized that these days we can ’ t think of the world we now call the middle east without thinking of it as islamic . perhaps the greatest testimony to islam ’ s power to organize peoples lives and their communities is that , in egypt , 5 times a day millions of people turn away from the pyramids and toward mecca . egypt , birthplace to one of the longest continuous cultures the world has ever known , is now the largest arab country in the world . next week we ’ ll talk about the dark ages . spoiler alert : they were darkest in the evening . thanks for watching and we ’ ll see you next time . crash course is produced and directed by stan muller , our script supervisor is danica johnson . the show is written by my high school history teacher raoul meyer and myself and our graphics team is thought bubble . last week ’ s phrase of the week was “ they might be giants ” . if you want to guess this week ’ s phrase of the week or suggest future ones you can do so in comments where you can also ask questions about today ’ s video that our team of historians will endeavor to answer . thank you so much for watching and as they say in my hometown , don ’ t forget to be awesome .

[ theme music ] so in less than 200 years islam went from not existing to being the religious and political organizing principal of one of the largest empires in the world . and that story begins in the 7th century ce when the angel gabriel appeared to muhammad , a 40-ish guy who made his living as a caravan trader and told him to begin reciting the word of god . initially , this freaked muhammad out , as , you know , it would—but then his wife and a couple of other people encouraged him and slowly he came to accept the mantle as prophet .

which angel appeared to muhammad ?

where does bread get its fluffiness ? swiss cheese its holes ? and what makes vinegar so sour ? these foods may taste completely different , but all of these phenomena come from tiny organisms chowing down on sugar and belching up some culinary byproducts . let 's start with yeast . yeast are single-celled fungi used to make bread , beer , and wine , among other products . yeast break down carbohydrates , like sugar , to get energy and the molecules they need to function . they have two different ways to do this : the oxygen-dependent , or aerobic , pathway , and the oxygen-independent , anaerobic pathway , which is also called fermentation . when you bake bread , yeast can use both pathways , but they normally prefer to start with the anaerobic process of fermentation . in this process , ethanol is produced in addition to co2 . no , bread is n't alcoholic . small amounts of alcohol that are secreted evaporate during baking . in the aerobic , or oxygen-dependent pathway , the yeast consume some of the sugar and produce carbon dioxide gas , or co2 , and water . in both processes , the co2 accumulates and creates tiny bubbles . these bubbles get trapped by gluten and create a sponge-like structure that gives the bread its soft texture . wine also relies on yeast . but a wine-making set-up keeps the oxygen levels low so that yeast consume sugar using fermentation , the anaerobic pathway . the process often starts with wild yeasts already hanging out on the grapes . but to get consistent results , most winemakers also add carefully selected strains of yeast that can tolerate high levels of alcohol . the yeast consume the sugar in the grape juice , and as the sugar level drops , the alcohol level rises . this does n't necessarily mean that sweeter wines have less alcohol . different types of grapes start with different amounts of sugar , and sugar can also be added . what happens to the carbon dioxide ? it just bubbles away through a vent . in carbonated alcoholic beverages , like champagne and beer , sealed containers are used in primary or secondary fermentation to keep the carbon dioxide in the bottle . wine also introduces us to our second type of food-producing microorganism : bacteria . a special strain of bacteria turns a tart compound in grape juice into softer tasting ones that are responsible for some of the flavors in red wines and chardonnays . another type of bacteria , called acetic acid bacteria , is n't so desirable in wine , but they have their function , too . if there 's oxygen around , these bacteria convert the ethanol in wine into , well , acetic acid . let this process continue and you 'll eventually get vinegar . bacteria are the key for cheese , too . to make cheese , milk is inoculated with bacteria . the bacteria gobble up the lactose , a kind of sugar , and produce lactic acid , along with many other chemicals . as the milk gets more and more acidic , its proteins start to aggregate and curdle . that 's why spoiled milk is clumpy . cheesemakers usually add an enzyme called rennet , naturally found inside of cows , goats , and some other mammals to help this process along . eventually , those little curdles turn into bigger curds , which are pressed to squeeze out the water , and create a firm cheese . different strains of bacteria make different kinds of cheese . for example , a species of bacteria that emits carbon dioxide is what gives swiss cheese its characteristic holes . some cheeses , brie and camembert , use another kind of microorganism , too : mold . so your kitchen functions as a sort of biotechnology lab manned by microorganisms that culture your cuisine . yogurt , soy sauce , sour cream , sauerkraut , kefir , kimchi , kombucha , cheddar , challah , pita , and naan . but maybe not all at the same dinner .

where does bread get its fluffiness ? swiss cheese its holes ?

what conditions do you need to change in order for the bread to become fluffier ? name at least three .

thallium , because it is heavier , has two different so-called oxidation states . it can react with one atom of chlorine per thallium , or say three , and the different chemistry between thallium-one and thallium-three . thallium salts are very poisonous , and there was quite a notorious case some years ago when a technician at some company , who turned out to be mad , was discovered to have been poisoning many of his colleagues with thallium and so , but it is not a widely used element , apart from a few materials that are used . there is a rather nice red material that is made of thallium , which has the rather unromantic name of krs-5 , this is mixed bromide and iodine with thallium which is used as windows for many infrared experiments . but because thallium is so poisonous , one has to be quite careful about handling .

thallium , because it is heavier , has two different so-called oxidation states . it can react with one atom of chlorine per thallium , or say three , and the different chemistry between thallium-one and thallium-three . thallium salts are very poisonous , and there was quite a notorious case some years ago when a technician at some company , who turned out to be mad , was discovered to have been poisoning many of his colleagues with thallium and so , but it is not a widely used element , apart from a few materials that are used .

unlike the other elements in its family , thallium can exist in two oxidation states . one of them is +3 . what is the other ?

currently i think there are eight intelligences that i ’ m very confident about and a few more that i ’ ve bene thinking about . i ’ ll share that with our audience . the first two intelligences are the ones which iq tests and other kind of standardized tests valorize and as long as we know there are only two out of eight it ’ s perfectly fine to look at them . linguistic intelligence is how well you ’ re able to use language . it ’ s a kind of skill that poets have , other kinds of writers , journalists tend to have linguistic intelligence , orators . the second intelligence is logical mathematical intelligence . as the name implies logicians , mathematicians , scientists have that kind of intelligence . they ’ re able to do mathematical proofs . they ’ re able to do scientific reasoning and experimentation . and it ’ s great to have language and logical intelligence because most tests really focus on that . and if you do well in those tests as long as you stay in school you think you ’ re smart . but if you ever walk out into broadway or the highway or into the woods or into a farm you then find out that other intelligences are at least this important . so the third intelligence is musical intelligence and that ’ s the capacity to appreciate different kinds of musics , to produce the music by voice or by an instrument or to conduct music . and people say well music is a talent . it ’ s not an intelligence . and i say well why if you ’ re good with words is that an intelligence but if you ’ re good with tones and rhythms and timbres nobody ’ s ever given me a good answer which is why it makes sense to talk about musical intelligence . and at certain cultures over history musical intelligence has been very important . the fourth intelligence is spatial intelligence . that ’ s the intelligence which allows us to handle and work in space that ’ s close by . a chess player would have spatial intelligence . a surgeon would have spatial intelligence . but there ’ s another variety of spatial intelligence which we use for a much broader navigation . that ’ s what an airplane pilot or a sea captain would have . how do you find your way around large territory and large space . similarly with the fifth intelligence bodily kinesthetic intelligence it comes in two flavors . one flavor is the ability to use your whole body to solve problems or to make things . and athletes and dancers would have that kind of bodily kinesthetic intelligence . but another variety is being able to use your hands or other parts of your body to solve problems or make things . a craft person would have bodily kinesthetic intelligence even if they weren ’ t particularly a good athlete or dancer . the sixth intelligence and seventh intelligence have to do with human beings . interpersonal intelligence is how you understand other people , how you motivate them , how you lead them , how you work with them , how you cooperate with them . anybody at any workplace with other people needs interpersonal intelligence . leaders hopefully have a lot of interpersonal intelligence . but any intelligence can be used in a pernicious way so the salesman that sells you something you don ’ t want for a price you don ’ t want to pay , he or she has got interpersonal intelligence . it ’ s just not being used in a way that we might prefer . the seventh kind of intelligence is difficult to assess but it ’ s very important . it ’ s intrapersonal intelligence . it ’ s the understanding yourself . if we go back a way in history and prehistory knowledge of yourself probably wasn ’ t that important because people did what their parents or grandparents did whether they were hunters or fisherman or craftspeople . but nowadays especially in developed society people lead their own lives . we follow our own careers . we often switch careers . we don ’ t necessarily live at home as we get older . and if you don ’ t have a good understanding of yourself you are in big trouble . so that ’ s intrapersonal intelligence . the eighth intelligence which i added some years ago is the naturalist intelligence . and that ’ s the capacity to make important relevant discriminations in the world of nature between one plant and another , between one animal and another . it ’ s the intelligence of the naturalist , the intelligence of charles darwin . i missed it the first go around when i wrote about it but i tried to atone by adding it to my list . and by the way you might say well but nature isn ’ t so important anymore . but in fact everything we do in the commercial world uses our naturalist intelligence . why do i buy this jacket rather than another one ? this sweater rather than another one ? one hair style rather than another ? those all make just the naturalist intelligence because the brain is very adaptive . and when an old use of a brain center no longer is relevant it gets hijacked for something new . so we ’ re all using our naturalist intelligence even if we never walk out into the woods or into the savannah of east asia . the two other intelligences which i ’ m interested in , one of them is called the teaching or pedagogical intelligence . the intelligence which allows us to be able to teach successfully to other people . now you could have two people who have exactly the same expertise and knowledge in the field but one is a very good teacher and the other isn ’ t . that probably doesn ’ t surprise individuals so much . but what got me fascinated was as young as two or three kids already know how to teach . now what does that mean ? you show a child how to do something let ’ s say a three or four year old and then you ask the child to explain it to an older person or to a younger person . and even the three or four year old will explain it very differently to a young person , will go through details , point things and speak slowly . and with an older person it would be much more elliptical and say well you do this and that and then you can figure it out . so that shows as young as three let ’ s say we already have teaching intelligence . the other one is one which i think is going to be difficult to prove to a skeptic but i call it existential intelligence . and existential intelligence is the intelligence of big questions . philosophical questions , artistic questions . what does it mean to love ? why do we die ? what ’ s going to be in the future ? my pet bird might have more musical intelligence . the rats who are scurrying around the floor might have more spatial intelligence . but no other animals have existential intelligence . part of the human condition is to think about questions of existence . and i like to say every five year old has existential intelligence because five year old are always asking why this , why that . but the difference between a five year old and a philosopher is the five year old doesn ’ t pay too much attention to the answer whereas philosophers and other people who develop existential intelligence are really very interested in how we attack questions like that . so again where there ’ s eight intelligences or ten or twelve is less important to me than having broken the monopoly of a single intelligence which sort of labels you for all time . i think if we lived forever we could probably develop each intelligence to a very high degree . but life is very short and if you devote too much attention to one intelligence you ’ re not going to have much time to work on other kinds of intelligences . and so the big question is should you play to strength or should you bolster weakness ? and that ’ s a value judgment . scientists can not give you an answer to that . if , for example , you want to be a jack of all trades and be very well rounded then probably you ’ re going to want to nurture the intelligences which aren ’ t that strong . if on the other hand you ’ re dead set on really coming to the top of some particular heap then you ’ re probably going to find the intelligences that you ’ re strongest at and really push those . and , you know , if a parent came to me and said well should we supplement or should we accentuate i would say well tell me what you would like your child to do . or better let the child tell you what he or she wants to do rather than say well science says you should do one or the other . i think it ’ s a question of values , not of science . some people think there ’ s such a thing as humor intelligence . but , in fact , i don ’ t . i think humor intelligence is simply the operation of a logical intelligence in some realm like human nature or physical nature or the workplace . and what happens is in humor there ’ s a certain expectation and you flip that expectation so it ’ s logic but it ’ s logic that ’ s played out in different kinds of ways . people had mentioned there ’ s such a thing as a cooking intelligence , a humor intelligence and a sexual intelligence . and i quipped well that can ’ t be intelligences because i don ’ t have any of them .

the seventh kind of intelligence is difficult to assess but it ’ s very important . it ’ s intrapersonal intelligence . it ’ s the understanding yourself .

how is intrapersonal intelligence different than interpersonal intelligence ?

it 's so obvious that it 's practically proverbial . you ca n't unboil an egg . well , it turns out you can , sort of . what thermal energy does to the eggs ' molecules , mechanical energy can undo . eggs are mostly made of water and proteins . the proteins start off folded up into intricate shapes , held together by weak chemical bonds . adding heat disrupts those bonds , allowing the proteins to unfold , uncoil , unwind and wiggle freely . this process is called denaturing . the newly liberated proteins bump up against their neighbors and start to form new bonds with each other , more and more as the heat increases , until finally , they 're so entangled that they gel into a solid mass , a boiled egg . that entanglement might look permanent , but it 's not . according to a chemical idea called the principle of microscopic reversibility , anything that happens , like egg proteins seizing up , can theoretically unhappen if you retrace your steps . but adding more heat will tangle the proteins further , and cooling them down will only freeze them , so here 's the trick : spin them around ridiculously fast . i 'm not kidding . here 's how it works . first , scientists dissolve boiled egg whites in water with a chemical called urea , a small molecule that acts as a lubricant , coating the proteins ' long strands and making it easier for them to glide past each other . then , they spin that solution in a glass tube at a breakneck 5000 rotations per minute , making the solution spread out into a thin film . here 's the key part . the solution nearest the wall spins faster than the solution closer to the middle . that difference in velocity creates sheer stresses that repeatedly stretch and contract the proteins until eventually they snap back into their native shapes and stay there . by the time the centrifuge stops spinning , the egg white is back in its original unboiled state . this technique works with all sorts of proteins . bigger , messier proteins can be more resistant to being pulled apart , so scientists attach a plastic bead to one end that adds extra stress and encourages it to fold up first . this unboiling method wo n't work with a whole egg in its shell since the solution has to spread throughout a cylindrical chamber . but the applications go way beyond uncooking your breakfast , anyhow . many pharmaceuticals consist of proteins that are extremely expensive to produce , partly because they get stuck in tangled up aggregates , just like cooked egg whites and have to be untangled and refolded before they can do their jobs . this spinning technique has the potential to be an easier , cheaper and quicker method than other ways to refold proteins , so it may allow new drugs to be made available to more people faster . and there 's one more thing you need to keep in mind before trying to uncook all of your food . boiling an egg is actually an unusual cooking process because even though it changes the way proteins are shaped and bound together , it does n't actually change their chemical identity . most types of cooking are more like the famous maillard reaction , which makes chemical changes that turn sugars and proteins into delicious caramel crunchiness and are a lot harder to undo . so you might be able to unboil your egg , but i 'm sorry to say you ca n't unfry it ... yet .

eggs are mostly made of water and proteins . the proteins start off folded up into intricate shapes , held together by weak chemical bonds . adding heat disrupts those bonds , allowing the proteins to unfold , uncoil , unwind and wiggle freely .

a folded protein is held in place by _____

there was a time before our ancestors smashed flint and steel together , when they felt the cold lack of fire in their lives . but anthropologists theorize that early hominids relied on lightning to cause forest fires , from which they could collect coals and burning sticks . fire gave them the ability to cook food and clear land , and became central in many rituals and traditions . so instead of seeing forest fires as an exclusively bad thing , ancient humans may have learned to appreciate them . yet , it was n't just humans who benefitted from these natural phenomena . even as they destroy trees , fires also help the forest themselves , however counterintuitive that seems . in fact , several forest species , such as select conifers , need fire to survive . but how can fire possibly create life in addition to destroying it ? the answer lies in the way that certain forests grow . in the conifer-rich forests of western north america , lodgepole pines constantly seek the sun . their seeds prefer to grow on open sunny ground , which pits saplings against each other as each tries to get more light by growing straighter and faster than its neighbors . over time , generations of slender , lofty lodgepoles form an umbrella-like canopy that shades the forest floor below . but as the trees ' pine cones mature to release their twirling seeds , this signals a problem for the lodgepoles ' future . very few of these seeds will germintate in the cool , sunless shade created by their towering parents . these trees have adapted to this problem by growing two types of cones . there are the regular annual cones that release seeds spontaneously , and another type called serotinous cones , which need an environmental trigger to free their seeds . serotinous cones are produced in thousands , and are like waterproofed time capsules sealed with resinous pitch . many are able to stay undamaged on the tree for decades . cones that fall to the ground can be viable for several years , as well . but when temperatures get high enough , the cones pop open . let 's see that in action . once it 's gotten started , a coniferous forest fire typically spreads something like this . flames ravage the thick understory provided by species like douglas fir , a shade-tolerant tree that 's able to thrive under the canopy of lodgepole pines . the fire uses these smaller trees as a step ladder to reach the higher canopy of old lodgepole pines . that ignites a tremendous crown fire reaching temperatures of up to 2400 degrees fahrenheit . that 's well more than the 115-140 degrees that signal the moment when serotinous seeds can be freed . at those temperatures , the cones burst open , releasing millions of seeds , which are carried by the hot air to form new forests . after the fire , carbon-rich soils and an open sunlit landscape help lodgepole seeds germinate quickly and sprout in abundance . from the death of the old forest comes the birth of the new . fires are also important for the wider ecosystem as a whole . without wildfires to rejuvenate trees , key forest species would disappear , and so would the many creatures that depend on them . and if a fire-dependent forest goes too long without burning , that raises the risk of a catastrophic blaze , which could destroy a forest completely , not to mention people 's homes and lives . that 's why forest rangers sometimes intentionally start controlled burns to reduce fuels in order to keep the more dangerous wildfires at bay . they may be frightening and destructive forces of nature , but wildfires are also vital to the existence of healthy boreal forest ecosystems . by coming to terms with that , we can protect ourselves from their more damaging effects while enabling the forests , like the legendary phoenix , to rise reborn from their own ashes .

the answer lies in the way that certain forests grow . in the conifer-rich forests of western north america , lodgepole pines constantly seek the sun . their seeds prefer to grow on open sunny ground , which pits saplings against each other as each tries to get more light by growing straighter and faster than its neighbors . over time , generations of slender , lofty lodgepoles form an umbrella-like canopy that shades the forest floor below .

lodgepole pine seedlings prefer to grow on ________ , sunny ground .

would you believe that walruses , rattlesnakes , and parakeets all once lived in the same house ? let 's go back about 350 million years . look around . steamy swamps and rain forests of horsetails and ferns cover the region . amphibians are the dominant land vertebrates . they range in size from newts to crocodiles . and all require water to do their egg laying . if they do n't go to the water , their shell-less , jelly-like eggs will dry out . because of this hazard , they spend most of their time living in or near fresh water . that is , until a breakthrough in evolution changes everything : the amniotic egg . the amniotic egg is shelled , waterproof , and can be laid on dry land . it is produced by the amniotes , a new group of animals named after their revolutionary egg . the first amniote is a tetrapod , a four-legged animal , resembling a small lizard . while some amphibians can walk around on land and bury their eggs in wet soil or highly humid areas , nothing before the amniotes has the ability to lay its eggs on completely dry land . because of this evolved egg , the amniotes are the first animals with the ability to live a fully terrestrial life . but , despite their move inland , the amniotes have not abandoned their pond-dwelling upbringing . in fact , the amniotic egg brings the pond with them by enclosing the aquatic environment within its shell . this is achieved by four main upgrades that are unique to amniotic eggs . let 's take a closer look . the first development is the most obvious : the egg 's protective shell . it 's tough but flexible , and has a leathery surface , still seen in reptile eggs today . the shell protects the eggs from predators , bacteria , damage , and drying out . but , unlike the walls of a fish tank , the shell of the amniotic egg is porous , allowing oxygen to pass through so that the growing amniote inside does n't suffocate . the next two developments are two separate membranes that work together like a pair of lungs . they bring oxygen into the embryo while removing carbon dioxide . the first is the chorion , which is the protective layer that oxygen passes through after entering the shell 's tiny pores . you may recognize the chorion as the thin skin you peel away on a hard boiled egg . think of this waterproof membrane as the in and out doors of the egg . it 's the entrance for oxygen and exit for carbon dioxide . the membrane working with the chorion is the allantois . if the chorion is the doors , then the allantois is essentially the lobby of the building . it directs the oxygen and carbon dioxide while simultaneously storing unneeded waste from the embryo . the chorion and the allantois make sure the embryo has everything it needs and gets rid of anything it does n't . the last and perhaps the most important development is the amnion , the membrane for which the egg is named . the amnion is also contained within the chroion and holds the fluid in which the embryo floats . because it has left the watery world of the amphibians , the amnion is necessary for preventing the embryo from drying out . it is the transportable pond that allows the amniote to lay the egg on dry land . its fluid also protects the embryo from any collisions or rough landings , like a shock absorber on your bike or car . together , the shell and these four membranes create a safe , watery environment for the embryo to grow and develop . the new amniote offspring will continue the process of vertebrate evolution as it explores new land away from the water . they will spend the next million years splitting into two distinct groups : the synapsids and sauropsids . synapsida is the group of animals that contain mammals , while sauropsida is the group that contains reptiles , birds , and dinosaurs . these two amniotic groups collectively contain the walruses , rattlesnakes , and parakeets we know today . like a family reunion , with relatives of every shape and size , coming together from different corners of the earth , these animals can all call one place home : the amniotic egg .

they will spend the next million years splitting into two distinct groups : the synapsids and sauropsids . synapsida is the group of animals that contain mammals , while sauropsida is the group that contains reptiles , birds , and dinosaurs . these two amniotic groups collectively contain the walruses , rattlesnakes , and parakeets we know today .

mammals belong to the group known as :

what gives the trumpet its clarion ring and the tuba its gut-shaking `` omm pah pah ? '' and what makes the trombone so jazzy ? the answer lies not in the brass these instruments are made of , but in the journey air takes from the musician 's lungs to the instrument 's bell . like any sound , music consists of vibrations traveling through air . instruments are classified based on how those vibrations are produced . percussion instruments are struck . string instruments are plucked or bowed . woodwinds have air blown against a reed or sharp edge . for brass instruments , however , the vibration come directly from the musician 's mouth . one of the first things a brass player must learn is to breathe in deeply , until every possible particle of air is crammed into the lungs . once all that air is inside , it must come out through the mouth , but there , an internal battle takes place as the musician simultaneously tries to hold their lips firmly closed while blowing enough air to force them open . the escaping air meets resistance from the lip muscles , forms an opening called the aperture and creates the vibration that brass players call `` the buzz . '' when a mouthpiece is held up to those vibrating lips , it slightly refines the buzz , amplifying the vibration at certain frequencies . but things get really interesting depending on what instrument is attached to that mouthpiece . a brass instrument 's body is essentially a tube that resonates with the air column blowing through it . the way that sound waves travel through this column forms a limited pattern of pitches known as the harmonic series , with notes spaced far apart at the lower end , but coming closer together as the pitch increases . the musician can alter the pitch of the note through slight contractions of the lips and alterations to air volume and speed . slower , warm sighing air produces lower pitches , and faster , cool , flowing air produces higher pitches in the series . but any single harmonic series has gaps where pitches are missing and the versatility of brass instruments lies in their ability to switch between multiple series . on instruments like the trumpet , valves can be lowered to increase the length of tubing the air travels through , while on a trombone , this is done by extending its slide . lengthening the tube stretches the vibrating air column , reducing the frequency of vibrations and resulting in a lower pitch . this is why the tuba , the largest brass instrument , is also the one capable of playing the lowest notes . so changing the instrument length shifts its harmonic series , while slight variations of the air flow and the player 's lips produce the different notes within it . and those notes finally emerge through the flared bell opening at the end . what started as a deep breath and a vibrating buzz on the lips has now been transformed into a bold and brassy tune . the musician 's skillful manipulation of every part of the process from lungs , to lips , to the mouthpiece , to the instrument itself creates an amazing palette of pitches that can be heard in musical genres across the globe . by harnessing the power of natural resonance in a flexible and controllable way , brass instruments are great examples of the fusion of human creativity with the physics of our world .

once all that air is inside , it must come out through the mouth , but there , an internal battle takes place as the musician simultaneously tries to hold their lips firmly closed while blowing enough air to force them open . the escaping air meets resistance from the lip muscles , forms an opening called the aperture and creates the vibration that brass players call `` the buzz . '' when a mouthpiece is held up to those vibrating lips , it slightly refines the buzz , amplifying the vibration at certain frequencies .

the escaping air meets resistance from the lip muscles , which forms a hole called the _____ , and creates the vibration that brass players call the buzz .

have you experienced déjà vu ? it 's that shadowy feeling you get when a situation seems familiar . a scene in a restaurant plays out exactly as you remember . the world moves like a ballet you 've choreographed , but the sequence ca n't be based on a past experience because you 've never eaten here before . this is the first time you 've had clams , so what 's going on ? unfortunately , there is n't one single explanation for déjà vu . the experience is brief and occurs without notice , making it nearly impossible for scientists to record and study it . scientists ca n't simply sit around and wait for it to happen to them -- this could take years . it has no physical manifestations and in studies , it 's described by the subject as a sensation or feeling . because of this lack of hard evidence , there 's been a surplus of speculation over the years . since emile boirac introduced déjà vu as a french term meaning `` already seen , '' more than 40 theories attempt to explain this phenomenon . still , recent advancements in neuroimaging and cognitive psychology narrow down the field of prospects . let 's walk through three of today 's more prevalent theories , using the same restaurant setting for each . first up is dual processing . we 'll need an action . let 's go with a waiter dropping a tray of dishes . as the scene unfolds , your brain 's hemispheres process a flurry of information : the waiter 's flailing arms , his cry for help , the smell of pasta . within milliseconds , this information zips through pathways and is processed into a single moment . most of the time , everything is recorded in-sync . however , this theory asserts that déjà vu occurs when there 's a slight delay in information from one of these pathways . the difference in arrival times causes the brain to interpret the late information as a separate event . when it plays over the already-recorded moment , it feels as if it 's happened before because , in a sense , it has . our next theory deals with a confusion of the past rather than a mistake in the present . this is the hologram theory , and we 'll use that tablecloth to examine it . as you scan its squares , a distant memory swims up from deep within your brain . according to the theory , this is because memories are stored in the form of holograms , and in holograms , you only need one fragment to see the whole picture . your brain has identified the tablecloth with one from the past , maybe from your grandmother 's house . however , instead of remembering that you 've seen it at your grandmother 's , your brain has summoned up the old memory without identifying it . this leaves you stuck with familiarity , but no recollection . although you 've never been in this restaurant , you 've seen that tablecloth but are just failing to identify it . now , look at this fork . are you paying attention ? our last theory is divided attention , and it states that déjà vu occurs when our brain subliminally takes in an environment while we 're distracted by one particular object . when our attention returns , we feel as if we 've been here before . for example , just now you focused on the fork and did n't observe the tablecloth or the falling waiter . although your brain has been recording everything in your peripheral vision , it 's been doing so below conscious awareness . when you finally pull yourself away from the fork , you think you 've been here before because you have , you just were n't paying attention . while all three of these theories share the common features of déjà vu , none of them propose to be the conclusive source of the phenomenon . still , while we wait for researchers and inventers to come up with new ways to capture this fleeting moment , we can study the moment ourselves . after all , most studies of déjà vu are based on first-hand accounts , so why ca n't one be yours ? the next time you get déjà vu , take a moment to think about it . have you been distracted ? is there a familiar object somewhere ? is your brain just acting slow ? or is it something else ?

have you experienced déjà vu ? it 's that shadowy feeling you get when a situation seems familiar .

how many theories of déjà vu are there ?