Placeholder Image

Subtitles section Play video

  • We get asked a lot of questions here at SciShow.

  • Sometime we get a question that has maybe never been asked before in the history of

  • questions, and sometimes we get questions that are so universally wondered, that they

  • get asked over and over again.

  • So, we've compiled some of those frequent asks into one place, here, so hopefully, if

  • you've ever wondered these curious questions, you can get a whole bunch of answers right

  • now.

  • Recently, Patreon patron, Rob Margolis, reminded us of two of these questions that come up

  • a lot.

  • The first, I hope you're not wondering right now, but if you are, I hope you recover quickly

  • and can watch this video about what causes migraines.

  • If you've never had a migraine, you might think it's just a really bad headache.

  • But if you've ever had them, or you know someone who does, you know that they're

  • much worse -- and much more complicated -- than that.

  • A true migraine is a multi-symptom disorder of the central nervous system that affects

  • the brain.

  • But, yes, really bad headaches are a major component of it -- probably the single most

  • significant and identifiable component.

  • But it usually lasts longer than a normal headache -- anywhere from 4 hours to several

  • days// -- and brings a whole array of other symptoms with it.

  • Most migraine sufferers experience extreme sensitivity to light and sound, and sometimes

  • smells.

  • They also commonly experience nausea, vomiting, even fainting.

  • What little relief they can find is generally only achieved by being very still in a dark,

  • silent room until the symptoms pass.

  • And believe it or not, it gets worse.

  • Migraines also cause problems both before and after the headache.

  • It's different for everyone, but the ordeal can start with symptoms as seemingly minor

  • as constipation, weird food cravings, neck stiffness, or excessive yawning.

  • As the symptoms worsen, people generally enter a phase called aura, in which they may experience

  • things like vision disturbances -- like seeing shapes or lights, blurred or doubled vision,

  • or even loss of vision -- “pins and needlessensations in the extremities, weakness, and

  • sometimes even slurred speech.

  • Now, you might notice that these sound a lot like the symptoms of a stroke, and in fact

  • migraines have so many things in common with strokes that doctors sometimes have to do

  • tests to determine which disorder they're dealing with.

  • After the headache has passed, most migraine sufferers experience a period of weakness

  • and fatigue that can last from a few hours to a few days.

  • Obviously this isn't the sort of thing that anyone wants to experience.

  • So what causes it?

  • Can it be controlled?

  • Or at least treated?

  • Doctors think migraines are probably caused by a sharp drop in your brain's levels of

  • serotonin -- a neurotransmitter that plays a key role in regulating things like sleep

  • and mood.

  • And once that imbalance strikes, it causes a whole cascade of effects.

  • But what triggers this imbalance is complicated and uncertain.

  • We do know that one of the most important factors is genetics.

  • If one or both of your parents has experienced a migraine, odds are that you will too.

  • For reasons that we don't understand, women are far more likely to have migraines than

  • men, and they're even more likely to experience one during times of hormonal changes, like

  • puberty, menstruation, ovulation, pregnancy, when using hormonal contraceptives or hormone

  • replacements, and menopause.

  • Beyond that, everyone's triggers are different.

  • For many people, it may depend on stress, their activity level or their sleep schedule

  • -- all things in which serotonin plays a role.

  • And still others may be triggered by things as seemingly random as bright lights, loud

  • sounds, unusual or strong smells, or even weather changes.

  • The //treatment// of migraines is further evidence that it's not just a headache.

  • It's true that the headache itself can sometimes be treated with pain relievers, although they're

  • often less effective.

  • In addition to pain relief, migraine sufferers may take medications that try to treat the

  • source of attacks, like by controlling the constriction of blood vessels in the brain,

  • blood pressure, serotonin levels, and inflammation.

  • So clearly a migraine is more than just a bad headache, remember that when you hang

  • out with people who get them.

  • If they're in a bad way, the biggest favor you can give them is just to let them be by

  • themselves in a dark room.

  • You can just keep watching SciShow //quietly//.

  • Rob's second question is another that comes up a lot, but is less painful...for humans

  • anyway.

  • Welcome to I Don't Think It Means What You Think It Means, where we look at bits of scientific

  • theory that've wiggled their way into popular culture and taken on a life of their own.

  • Today we're talking about Schrodinger's Cat, a famous thought experiment devised by

  • Austrian physicist Erwin Schrodinger, who helped piece physics back together after Einstein

  • and his crew blew a giant honkin' hole in it back in the early 20th century.

  • It can't really be overstated how much of a giant crap circus the 1920's were for

  • physicists.

  • Until then, everything had pretty much just been good old-fashioned Newtonian physics

  • -- where you could observe objects moving, and predict how they'd react to various

  • forces.

  • But then along came new research into subatomic particles that showed they didn't act predictably

  • at all.

  • In fact, sometimes stuff seemed to be two things at once.

  • Like, an electron in a beam might act like a particle sometimes and like a wave at other

  • times.

  • And to make things even more -- [heaves tense sigh, sort of like hyperventilating]-- the

  • more you try to observe and measure these particles, the less naturally they seem to

  • behave.

  • Sphincter-say-what, now?

  • [js: Um, it's from Wayne's World and I think I'm trying to bring it back.]

  • My friends, welcome to one of the biggest mind-flogs of quantum mechanics; it's called

  • superposition -- the idea that a particle can exist in all of its theoretically possible

  • states at the same time.

  • So Schrodinger came up with this thought experiment to help folks understand it: Say you have

  • a cat and you put it in a steel chamber for an hour with a vial of deadly gas, a Geiger

  • counter, a hammer, and a tiny bit of something radioactive.

  • OK just bear with me.

  • Now say there's a 50/50 chance that one of the radioactive atoms is going to decay

  • within that hour.

  • If one of the atoms decays, the Geiger counter is going to trigger the hammer, shattering

  • the vial of poisonous gas.

  • Really, Schrodinger?

  • This is not the best way to get people behind the idea of funding the sciences.

  • So, there's a 50% chance at the end of the hour that the vial has been broken and the

  • cat is dead, and an equally good chance that the vial hasn't broken and the cat's just

  • kickin' it, wondering what's for supper.

  • But, what's actually happening in the box?

  • According to quantum mechanics, any one of those radioactive atoms would be in a superposition

  • of being both decayed and not decayed at the same time.

  • Because that's how quantum objects act.

  • So then that decayed atom will have both killed and not killed the cat, right?

  • Well that's the logical conclusion but the cat isn't a quantum object.

  • The cat is a big normal thing that obeys old-fashioned Newtonian laws.

  • So it, just like ever other cat in history is either alive or dead.

  • Schrodingers point, at least one of them is that the object is subject to two separate

  • sets of laws that can't be reconciled.

  • In order to know whether the atom is decayed or not is to open the box as see if the cat

  • is dead.

  • But in quantum mechanics, the state of superposition can't be observed.

  • So when the evil mad scientist finally opens the chamber, to observe, the superposition

  • collapses once the outcome is ensured.

  • Today, Schrodinger's Cat is talked about as some undead zombie cat or discussed at

  • being dead and not dead, alive in the box.

  • But Schrodingers point wasn't to prove you can make a cat both alive and dead but instead

  • prove that the quantum world doesn't mesh well with the normal world.

  • Alternatively the point the universe is pretty freakin' weird.

  • There are other interpretations of quantum mechanics that resolve the paradox but none

  • of them are easy to test.

  • My favorite is of course theMany Worldsinterpretation that states at the end of the

  • experiment and at the end of the superposition, alternate universes are created.

  • But in this case, one in which the cat is alive and one in which the cat is dead.

  • And to be clear I don't like this interpretation because it's the most likely one, I like

  • it because it's such a excellent plot device for science fiction novels.

  • Dreaming is one of the weirdest thing we do.

  • I mean, I don't want to diminish all the other strange crap our bodies are capable

  • of, 'cause a lot of it is cracked out on so many levels.

  • But dreams are a special kind of crazy.

  • No matter how many dreams you have in your life, every once in a while you wake up like,

  • WHAT THE HELL WAS THAT?”

  • But as with everything else, science is helping us understand why we dream, what our brains

  • are up to when they do it, and why dreaming may be critically important to the functioning

  • of our awake brains.

  • Try to stay awake for this, 'cause it's really cool.

  • People have been trying to understand dreams since--well, since there've been people.

  • But the person we associate most with the science of dreaming is probably Sigmund Freud.

  • In 1899 he wrote The Interpretation of Dreams, where he suggested that dreams were largely

  • symbolic and allowed us to sort through the repressed wishes that piled up in our unconscious

  • minds.

  • And most of those wishes involve weird sex stuff.

  • Freud was kinduva perv, if you must know.

  • It wasn't until the 1950s, when scientists became able to read the electrical activity

  • of the brain, that we began to understand what a dreaming brain was actually up to.

  • Two researchers at the University of Chicago -- Eugene Aserinsky and Nathaniel Kleitman

  • -- pioneered this research by hooking people up to the newly-invented EEG machine and monitoring

  • their brain activity while they slept.

  • What they thought they'd find was that a sleeping brain was a resting brain, but they

  • discovered exactly the opposite.

  • They found that brain activity fluctuates in a predictable pattern over a period of

  • about 90 minutes.

  • This cycle takes sleepers from an initial period of drifting off, gradually into a really

  • deep sleep with slower brain activity, back into almost-waking.

  • And this stage of sleep where the sleepers were aaaaalmost awake again was the most interesting:

  • brain activity in this phase was almost identical to when people were awake.

  • But even more weird, during this stage, the subjects became functionally paralyzed--the

  • only parts of their bodies that moved were their eyes, which darted back and forth under

  • their eyelids.

  • So Aserinsky and Kleitman called this period R.E.M. sleep, after the rapid eye movement

  • that characterized it.

  • They also called itparadoxical sleep,” because the subjects seemed to be awake, according

  • to their brain activity, even though they were basically dead to the world.

  • I guess they figured these names were better thanSexually Aroused Sleep,” which is

  • another rather common feature of this stage.

  • But another thing the scientists found was that if REM sleepers were awakened, they reported

  • having really vivid dreams that were often emotionally intense.

  • It wasn't the only stage of sleep in which the subjects dreamed, but it was the time

  • they reported having the most lifelike dreams.

  • It turns out that every 90 minutes or so, during the final stage of the sleep cycle,

  • the brain phases into the R.E.M. sleep and our brains start creating crazy narratives

  • that last maybe 20 or 30 minutes.

  • This is when you have those really vibrant dreams that can easily be confused with reality.

  • So WHYYYYY so busy, Sleeping Brain?

  • And what's so important about dreaming that you have to paralyze your entire body in order

  • to have really realistic dreams?

  • Well, there are probably several answers, but one of them is that during all periods

  • of dreaming, our brains are making important connections between real-life experiences

  • that will help us in our waking lives.

  • These days, researchers are finding that Freud was wrong about dreams in one important way:

  • We don't dream much about our hidden desires.

  • We mostly dream about what we did today.

  • While we sleep, our brains are sorting through what happened while we were awake, deciding

  • which new experiences were important enough to remember and which should get tossed, searching

  • for links between seemingly unrelated events that might be able to help us be a more successful

  • human tomorrow.

  • And it's actually really important that we do this while we're asleep, because our

  • conscious, waking brains are generally too controlling to allow this kind of creative

  • problem-solving.

  • And this dream-time activity helps our waking brains be better at things that require making

  • connections and thinking outside the box.

  • Dreams have actually been responsible for some really important inventions and discoveries

  • in history.

  • For instance, Dimitri Mendeleev came up with a system for the structure of the periodic

  • table of elements in a dream after months of grueling conscious thought was getting

  • him nowhere.

  • And research shows that our brains are much better at solving puzzles if they're allowed

  • to take a nap in the middle of doing one.

  • In a study in 2004, for instance, subjects were asked to search for links between two

  • sets of numbers.

  • The subjects who napped solved the puzzle about 60% of the time, whereas only 25% non-nappers

  • were able to do it.

  • In another study, where people were asked to find connections between seemingly unrelated

  • words, those who lapsed into R.E.M. sleep between sessions solved 40% more puzzles than

  • those who didn't.

  • So dreams are all about making associations and finding patterns that our waking brains

  • have a hard time detecting.

  • But it seems to work in slightly different ways in non-REM sleep than in REM sleep.

  • During non-R.E.M. sleep, you dream, but the dreams aren't necessarily vivid, and they're

  • often about something you've been doing or thinking about a lot.

  • During these stages, people often report dreaming about kind of boring stuff -- like if you

  • spent a lot of time in the car during the day, that night you might dream about driving

  • down a long street, stopping at a series of stop lights.

  • This might seem lame, but it's actually useful to the brain in its own way: it's

  • telling itself things it already knows--likewhen you're driving a car, you're supposed

  • to stop at the stop lights.”

  • So in non-REM sleep, it's basically reinforcing existing connections.

  • But in REM sleep, we get to test out that reinforced knowledge in a context that is

  • virtually indistinguishable from real life.

  • It's like our brain running simulations.

  • So if you've been driving to your grandparents' house in Boca Raton all day, and in non-REM

  • sleep you spend a good 20 minutes practicing stopping at traffic lights, during REM sleep

  • your brain might have you trying to steer a steamroller through Manhattan from the backseat.

  • REM dreams can be very lifelike and very stressful, but that's part of it: A vivid REM dream

  • is an opportunity to safely let us try something difficult.

  • Because our brains aren't here to make friends.

  • Our brains are here to win.

  • The evolutionary purpose of dreaming--like the evolutionary purpose of virtually everything

  • else we do--is to make us more successful animals tomorrow than we were yesterday.

  • So if during non-REM sleep, the brain is taking data from past experience and fiddling with

  • it to figure out how that might relate to the future, in REM sleep, the brain's actually

  • trying to experience the future in order to test possibilities.

  • So, maybe you're making out with your algebra teacher on Jay-Z's yacht while wearing a

  • banana suit.

  • What of it?

  • Does it mean you subconsciously want to make out with your teacher?

  • Maybe, but not necessarily.

  • Does the banana suit have something to do with penises?

  • I dunno - -who am I, Freud?

  • The thing is, during REM sleep you can try that experience out with no consequences whatsoever.

  • Another benefit of REM sleep is that it helps us process emotions that our dunderheaded

  • waking brains aren't really equipped to handle.

  • Although the content of our dreams might be wacky, the emotions attached to them are absolutely

  • real.

  • Remember, your dreaming brain is charged with working on real-life problems, so if you feel

  • really angry at your boyfriend in your dream, chances are you're probably pretty pissed

  • at him--or maybe someone else you're close to--in real life.

  • The stories our dreams create are essentially attempts to give our emotions a narrative

  • that can kind of suck the poison out of them and give them a form our brain can deal with

  • better.

  • In fact, people who can't experience REM sleep often experience other psychiatric disorders.

  • So, dreams help regulate traffic between our experiences, our emotions and our memories,

  • so we can dial down the crazy.

  • And hey--if the outcome makes your rational brain uncomfortable, well....

  • That's just how sausage is made, folks.

  • Since I'm on the topic of weird dreams and REM sleep, a lot of you have said you'd

  • like to know more about what's called lucid dreaming.

  • This is when you become aware of the fact that you're dreaming and can actually direct

  • the narrative of the dream.

  • Since REM sleep is simulation time in the brain, lucid dreaming is basically a simulation

  • that lets a portion of your conscious brain in on the action.

  • Most of us can probably recall at least one lucid dream, and about 1 in 10 of us have

  • them regularly.

  • Some lucid dreamers can even communicate with researchers studying them through gestures

  • like eye movements and hand-squeezes.

  • What ultimately separates lucid dreams from regular old REM sleep may lie in the physiology

  • of the brain.

  • During non-REM sleep, the cerebral cortex -- that's your gray matter -- loses its

  • ability to associate with other parts of the brain.

  • This is probably why those dreams are more boring and less complex.

  • But once a dreamer reaches REM sleep, the cortex becomes active again and begins talking

  • to other areas of the brain -- except for this one little part of the cortex--called

  • the dorsolateral prefrontal cortex -- that doesn't reactivate.

  • This is the region right about at your left temple that's responsible for, among other

  • things, applying memories to other situations, like planning stuff and predicting outcomes.

  • This helps explain why REM sleep dreams are so weird -- your brain literally can't tell

  • what's going to happen next.

  • But during lucid dreaming, the dorsolateral prefrontal cortex actually does wake up, which

  • is probably why we regain a sense of self-awareness and can plot out stories for ourselves.

  • Some people claim lucid dreaming can help cure reccurring nightmares or even help cure

  • depression and anxiety.

  • The jury's still out on that, but dreaming itself--all kinds of dreaming--is definitely

  • useful, and even imperative, to the function of the brain.

  • So, why are you still awake?

  • Go take a nap or something!

  • First thing you should know if you're having a hard time getting some shuteye, is that

  • you're wired to sleep regular hours...going to bed the same time each night and waking

  • up at the same time each morning.

  • Having a regular wake-up time seems to correlate pretty highly with the ability to fall asleep

  • consistently.

  • This is because it keeps you aligned with what's known as your circadian rhythm, your

  • body's natural tendency to stay in sync with the cycles of day and night.

  • And you know what controls your body's circadian rhythm more than anything?

  • Light.

  • A lot of the help you get falling asleep comes from hormones -- they lower your heart rate

  • and reduce your blood pressure and basically let you relax.

  • The key player here is the hormone melatonin, and it's regulated by your exposure to light.

  • In darkness, it flows freely.

  • But when you're exposed to light -- whether natural or artificial -- the release of melatonin

  • stops.

  • So you know what that means?

  • No phones or laptops in bed!

  • The light emitted by electronics simulates sunlight, and confuses your body into not

  • knowing that it's time to sleep.

  • So scientists suggest at least an hour of screen-free time before bed...though I am

  • completely incapable of that myself.

  • Another obvious enemy of sleep: caffeine [pic].

  • Even though you might think that cup of coffee after dinner might only affect you for an

  • hour or so, studies have shown that caffeine consumption as much as TWELVE HOURS before

  • bedtime is linked with insomnia.

  • And even the way you //think// about sleep can affect your sleep patterns.

  • Worrying about not getting enough sleep is a common enough cause of insomnia that it

  • has its own name, Sleep Onset Insomnia.

  • But you know what's weird?

  • A lot of the time, when we feel like we can't sleep -- we actually ARE sleeping.

  • When scientists rouse patients in the first or second stages of sleep, more than 60% of

  • them say that they weren't sleeping, even though they were.

  • Now, of course, there's a whole class of medications that will help you sleep, from

  • antihistamines to the pharmaceuticals known as hypnotics, which include Ambien and Lunesta.

  • However, research has shown that while patients [pic] //can// fall asleep faster on hypnotics,

  • the effect is small, adding only about 15 minutes to their sleep times.

  • Other studies indicate that //our minds// are significantly more powerful than any medications.

  • In double-blind studies, patients who were simply //told// that they were taking a sleep

  • drug ended up sleeping far better than patients who were told they weren't.

  • So, if you want to know how to sleep, the answer is right there in your head.

  • As part of our work answering the world's most asked questions, we asked you, our scishow

  • viewers, some questions...and one was how many hours per night you sleep.

  • Bad news: Only 10% of you are sleeping more than eight hours per night, and eight and

  • a half is the doctor-recommended amount.

  • And OVER HALF of you report having trouble getting to sleep at least once per week.

  • Oh but wait...we haven't gotten to the fun part...MEANINGLESS CORRELATIONS!!!

  • The best sleepers for countries where we had enough data to make a judgement were Saudi

  • Arabians, with 76 percent reporting that they experience insomnia infrequently or never.

  • Most of Europe scored better than average, with The Netherlands, Russia, and Spain all

  • sleeping relatively soundly.

  • The English speakers in the US, UK, and Australia all had some of the worst scores.

  • And, finally, unsurprisingly, our staggeringly unscientific survey reports that people who

  • commonly drink coffee, soda, energy drinks or tea are all more likely to suffer from

  • insomnia.

  • Though maybe they're just drinking those things because they're so tired!

  • So now that you know all about migraines, hypothetical cats, and sleep, on to the serious

  • questions!

  • Why?

  • WHY do we have baby teeth?

  • Missing teethnot so cute on the lead singer of the Pogues, but pretty dang cute on a smiling

  • toddler.

  • But why do humans have baby teeth, and why do we lose them?

  • Humans, like most mammals, are diphyodonts [dye-FYE-oh-donts], meaning we grow two sets

  • of teeth in our lifetimes -- a permanent set of adult teeth, and a deciduous set of baby

  • teeth.

  • Deciduous teeth are smaller and fewer in number, because a toddler's jaws are tiny and could

  • never fit a full set of 32 adult teeth.

  • Poor kids would look like something out of a horror movie.

  • So, instead, we begin life with 20 smaller teeth, which start erupting out of our gums

  • when we're about six months old and are fully in by the time we're two and a half.

  • Just like our permanent teeth, deciduous teeth grow in pairs.

  • Meaning that when two incisors erupt from the lower jaw, you can bet that two incisors

  • will soon erupt from the upper jaw.

  • This allows our mouths to bite down and chew evenly, and helps ensure that our jaws grow

  • and wear down evenly, too.

  • Now, as we get bigger, we need more teeth.

  • But instead of wedging these new teeth in between the old ones, we lose the old teeth

  • and grow a whole new set.

  • That's why baby teeth are called deciduous -- just like the leaves on deciduous trees,

  • they'll shed at a specific stage of development.

  • Four new molars erupt in the back of our mouths when we're around five or six years old.

  • Then our deciduous incisors -- which are right here [points to teeth in front of mouth] fall

  • out and are replaced with permanent incisors.

  • By the time we reach puberty, we have an almost-full set of 28 permanent teeth.

  • The last four emerge later in life.

  • These are the so-calledwisdom teeth,” molars in the back of our mouths.

  • They were a good idea some 100 million years ago when our jaws were bigger.

  • But evolution has made our mouths smaller, and now these molars crowd out other teeth

  • and can cause pain, which is why a lot of us get them pulled.

  • And being a diphyodont is actually kind of a disadvantage in other ways, too.

  • While it sounds nice to have an extra set of something, we only get two sets of teeth

  • in our lives.

  • Polyphyodonts [pol-ee-fye-o-donts], on the other hand, can grow and regenerate teeth

  • multiple times.

  • These include alligators, fish and even some mammals, like elephants, who can regenerate

  • their teeth up to six times, to help them enjoy long lives of grinding up plants.

  • But /our/ second set of teeth will just keep wearing and breaking over time, so take care

  • of them while you can, because they're the only ones you'll get!

  • Our final FAQ that we'll share with you today is one I'm pretty sure everyone has

  • wondered about whenever you've really stopped and thought about your eyebrows.

  • Or arm hair.

  • Or any hair.

  • You're probably quite happy that your armpit hair isn't dragging on the floor.

  • So it's good that there's a system to prevent that.

  • But what is that system?

  • Us humans grow hair all over our bodies -- except on our palms and the soles of our feet.

  • But some of it, like leg hair, stops growing, while the hair on our heads just seems to

  • grow out forever.

  • That's because every hair on your body goes through the same cycle -- growing for a while

  • and then falling out -- but each /type/ of hair spends a different amount of time growing,

  • and grows at a different speed.

  • Every hair begins the same way, in a phase of the cycle called anagen.

  • During anagen, blood flow starts to ramp up at the base of the follicle, feeding oxygen

  • to specialized stem cells.

  • These cells begin rapidly dividing and producing keratinocytes [ker-AT-in-oh-sites], which

  • form the root of the hair.

  • As the expanding mass of keratinocytes is pushed toward the surface of the skin, the

  • cells die, releasing a protein called keratin [CARE-a-tin], which holds the strand of hair

  • together.

  • Eventually that strand pops out of your skin.

  • So the visible part of the hair is entirely dead, which is why, thankfully, it doesn't

  • hurt to get your hair cut -- though try explaining that to a three-year-old.

  • During anagen, hair can grow up to 1 and a quarter centimeters every month, depending

  • on where it's located on your body.

  • The second phase is called catagen [CAT-a-jen], and lasts about two weeks.

  • Here, the blood supply is cut off at the bottom of the follicle, which stops the production

  • of new keratinocytes.

  • So for that particular hair, the party is over.

  • The follicle then shrinks to about a sixth of its original size, and the existing hair

  • strand is pushed closer the surface.

  • The third phase is called telogen [TELL-o-jen], otherwise known as the resting phase, where

  • the follicle remains dormant for one to four months.

  • Finally, the hair is released, or shed, when the follicle dilates, and starts the anagen

  • phase again.

  • So, how long a hair on your body gets, depends on how long it's in the anagen phase, and

  • how /fast/ it grows during that time.

  • The hair on your scalp, for example, stays in anagen for two to six years, which is why

  • it can grow so long.

  • Other hair types, like eyebrows and eyelashes and body hair have a short anagen phase -- only

  • 30 to 45 days.

  • But they also grow much more slowly, with eyebrows, for instance, growing only 4.2 millimeters

  • every month.

  • This is my eyebrow closeup.

  • Hey...

  • As for how your hairs know when to grow and when to stop, that's something scientists

  • are still trying to figure out.

  • It's known that genetics can lead to longer or shorter anagen phases in certain hair types.

  • But the current thinking is that your hairs get their instructions -- by way of chemical

  • growth signals -- from stem cells in the skin.

  • And considering how extremely inconvenient it would be for all of the hairs on your body

  • and all mammals' bodies to just continue growing forever, it makes sense that there's

  • a system for making sure they don't grow too long.

  • Thanks for watching this Frequently Asked Compilation video.

  • Please keep asking us questions!

  • You can ask in the comments, on Patreon, Tumblr, Facebook, Snapchat, email, via pigeon--if

  • you can figure that out, we'll have questions for you too!

  • And in the meantime, keep getting smarter with us by going to youtube.com/scishow to

  • subscribe.

We get asked a lot of questions here at SciShow.

Subtitles and vocabulary

Click the word to look it up Click the word to find further inforamtion about it