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How Does Hair Know When to Stop Growing?

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The hair on your scalp can get really long, but your leg hair doesn’t sweep the floor. Why not? Hosted by: Hank Green ---------- Dooblydoo thanks go to the following Patreon supporters -- we couldn't make SciShow without them! Shout out to Christopher Prevoe, Justin Ove, John Szymakowski, Peso255, Ruben Galvao, Fatima Iqbal, Justin Lentz, and David Campos. ---------- Like SciShow? Want to help support us, and also get things to put on your walls, cover your torso and hold your liquids? Check out our awesome products over at DFTBA Records: http://dftba.com/scishow Or help support us by becoming our patron on Patreon: https://www.patreon.com/scishow ---------- Looking for SciShow elsewhere on the internet? Facebook: http://www.facebook.com/scishow Twitter: http://www.twitter.com/scishow Tumblr: http://scishow.tumblr.com Instagram: http://instagram.com/thescishow Sources:http://www.webmd.com/skin-problems-and-treatments/hair-loss/science-hair?page=2 https://www.aad.org/dermatology-a-to-z/for-kids/about-hair/how-hair-grows http://www.loc.gov/rr/scitech/mysteries/grayhair.html http://physrev.physiology.org/content/81/1/449#ref-533 http://www.livestrong.com/article/109520-eyebrow-growth-rate/ http://stemcells.nih.gov/info/basics/pages/basics1.aspx http://physrev.physiology.org/content/81/1/449#ref-533 http://news.yale.edu/2011/09/01/yale-scientists-find-stem-cells-tell-hair-its-time-grow
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Text Comments (814)
Sneha Mishra (4 days ago)
be slow and speak
SepehrMirzaei (9 days ago)
I am a bit confused. If we are to cut hair that is already "fully grown", such as our arm hair, would it not grow back until the hair is shed from the follicle and a new one begins to grow?
Anthony Ong (18 days ago)
Question! How does our skin know where to grow hair?
TheFlamerwolf 12110 (21 days ago)
The way I see it is that kids and teens hair grows up over there face but as your adult your 4head is shown more why is that
D O M I N O - Y U R I (1 month ago)
I hate how my body works, I take a shower in the morning, wait a few hours, and the armpit hair is already visible again, and the pores where the hair on my legs come out of are also clearly visible. If i don't shave my leg hair for like 2 days, it will be so long. I can't even wear tank tops anymore
Just In (1 month ago)
If only the hair on my head knew how to not thin out... Thanks for that one dad.
does hair follicle come back after it dies by later??
Dave Barrack (2 months ago)
So what is the difference between body hair and fur? Both have a terminal length. Are they the same thing, but we just call them different things to make ourselves feel special and slightly further separated from animals?
Abhishek Shroff (2 months ago)
How to stop growing of hairs on leg forever
Glazzing Sun (2 months ago)
What if ur hair grows like 2 inch every month ;-;
Daniel Domino (3 months ago)
What happens when someone has a cast on one arm for a long period of time, and when it gets removed, the arm comes out a lot more hairy than the other?
Eight Great (3 months ago)
Why do i have hair growing on my neck in front my throat
Rhys Logan (3 months ago)
OK but if hair stops growing doesn't that make it fur?
Piano Girl (4 months ago)
I once accidentally shaved my eyebrows in 8th grade. Had to wear Sharpie for months lol.
GURLITSANASTASIJA Gurl (4 months ago)
I'm 12 and my hair grows so long:(
Klaus Hoxha (4 months ago)
But there’s this woman that has super long hair can someone explain that to me
Rayan Ighil (4 months ago)
Of course there is a system it's the big bang that made sure that our hair doesnt grow for ever what a great coincidence
Fae (4 months ago)
Geekish Goddess (5 months ago)
Why is my leg hair curly like my hair
Carlos Ramirez (5 months ago)
"How does hair know to grow... Does it touch your brain?"
Noob Gaming102 (5 months ago)
Did you know if you keep removing the hair from the root your body will stop that one strand from growing because it thinks it’s not needed anymore
craig ferguson (5 months ago)
God don’t play 🙏🏾
LydiA Jkessi (5 months ago)
I think my hair stop growing when it reaches bottom of my chest is that possible
mos ab (5 months ago)
So, when a hair in catagen or telogen is cut, it won't grow back?
Batabii (5 months ago)
But why does my scalp hair not seem to grow as fast as other people's? I can't seem to get it to reach waist-length
Ronald Borla (6 months ago)
Short answer: "We don't know" In my mind, what's the point of the video, then?
masterofthecontinuum (6 months ago)
when the hair detaches itself from the root, how is it still connected to the arrector pilli and able to stand up? and when the hair falls out, what happens with this muscle connection? does it detach and reattach to hairs as they grow, fall out, and are replaced?
ℛɛᴛʀᴏ ℛɛᴅ (6 months ago)
Here's another one: why do black people's hair grow slower than white people hair.
I Seek Revenge (6 months ago)
but why does my head hair stop growing at the point when it gets to my shoulders (am black)
Sean Peacock (6 months ago)
I have 2 individual eyebrow hairs that grow about twice as long as the rest. It is occasionally annoying.
Jimmy Joe (6 months ago)
I don't remember seeing my arm or leg hair fall out, only eyelashes, armpit/pubic and head hair.
Red Point (6 months ago)
So... No Rapunzel.... Allright...
GoldonLocks (6 months ago)
I have a hair on my arm that does not retract, I got it to 2 inches before, it’s easily removed with some scissors
BlackTec Playz /BT (6 months ago)
Evanston Conner (6 months ago)
I wish we can control hair groth by folicue
NWA362 (6 months ago)
Bruce Lee eyebrows was full and he also had a lot of hair on his head mean his genes was dope!! love how full his eyebrows were..
Shadow /Porcelain (6 months ago)
Im guessing in some cases it doesnt work well because ive had a single strand of arm hair be really long
Harsh Kukadia (6 months ago)
does plucking stop hair growth ?
UNSC INFINITY (6 months ago)
What about my Ass Hairs?? ... It's like a freakin Amazon Jungle down there!!
RAHUL (6 months ago)
Hello hank Green
goodman854 (6 months ago)
This didn't really answer the question for me. I want to know if I cut this hair on a mole if it will grow back
Space Cat 91c0 (7 months ago)
After a hair cut within three days you can tell my hair is growing back
DodgeMartin (7 months ago)
While I was watching this I couldn't stop looking at his hair
iZorohhh (7 months ago)
Well my eyebrows stopped growing for a while and one side of my eyebrow looks like it is shaved off...
Terren Maplethorpe (7 months ago)
I got a add for male pattern baldness for this video
darrick steele (8 months ago)
When people grow their hair as long as it will grow it usually tends to stop growing at about the waist line. But if they cut it back, it will grow again until it gets back to the waist. Some people can grow their head hair all the way past the ankles, I had a cousin with hair that long. Her hair seemed to just keep growing for ever. I am still wondering how hair knows that it's waist long.
Mfbsmd sifsai (8 months ago)
why does my hair grow faster in some areas?
Jognt (6 months ago)
Because it is genetically programmed to do so. Each hair on your body is pretty much unaware of the other hairs on your body. It just does its thing.
me tube (8 months ago)
Leongon draws stuff (8 months ago)
God damn it... the answer at the end of the video is "We don't know". I really wanted to know how the hair in arms and legs can know when to stop growing considering they start growing so fast after being shaved.
Genericname123 (9 months ago)
Dat close up!
Alexa Bella Muerte (9 months ago)
Rage Coder (9 months ago)
I thought they don't stop growing
adeQuate (9 months ago)
Then why do how do they know when to grow and how long when cut, it they are not connected?
SugarCofee (9 months ago)
''try explaining that to a 3yo'' hahahahahahhahahahahahhahahahahahahhaha
michael (10 months ago)
Why does women grow longer hair and more straight than men?
Jognt (6 months ago)
Because they don't. Men just cut it more frequently. Regarding curliness/straightness, the longer a hair becomes, the less likely it is to have significant curls. That's just gravity doing its thing.
Lolly Lemur (10 months ago)
1:01 Me at that age though
runnerJong (10 months ago)
why do some people grow pubic hair on their faces?
krishnaa balaji (10 months ago)
So this is why some people grow beards like more than a few inches, but some can't grow it longer than an inch or so?
sumedh deshpande (10 months ago)
Can you make a video on how hair loss serums and tablets work?
Mateolevic (10 months ago)
Intelligent design. Amen.
candlewax122 (11 months ago)
Okay, but how do I *stop* anagen?
Mike Altieri (11 months ago)
So basically you have NO IDEA why hair stop growing, click bait much?! This is supposed to be a science channel not a "we really don't know, we're just baiting to keep our numbers up"
Hana Silitonga (11 months ago)
So what about when you shave or cut your hair? Is it true that your hair will grow faster when you shave or cut your hair more often?
CeeJay Nor-Val (1 year ago)
I'm only 12 years old but I had a very long hair on legs
Julia George (1 year ago)
😩i will start form know we will going to see what is going to happen 🤔
starchild (1 year ago)
I think if you let your hair grow out as much as it can and just leave it be and not cut it your body will use the extra growth hormone or whatever that is not being used since ur hair has grown to its full potential for other things like repair other stuff muscles groups etc.. I dont know just a theory.
castleinlondon (1 year ago)
Keratin-o-cytes. Because they hold large amounts of keratin protein. Not kera-atin-cyte....
qthoshi (1 year ago)
Why do older men start growing longer eyebrow hairs, nose hairs and hair in their ears?
UriYah Ysrayl (1 year ago)
Answer: God
Jay-Rock (1 year ago)
I want my hair back please giving me a solution without that Bosley surgery.
Here I'm (1 year ago)
How can we kill *keratinocytes* on our body without _laser treatment?_
ram Bhagat (1 year ago)
why fish haven't hairs😯
Amarilis (1 year ago)
That made me chuckle "tho try explaining that to a 3 yr old" I CAN PICTURE THEM SCREAMING LIKE HELL IN A BARBER CHAIR!!!
Quirk91 (1 year ago)
Could there be a way to somehow keep the follicles dormant indefinitly? Or basically what im wondering is; could shaving pills be possible??? 8D
Stephanie Werst (1 year ago)
What causes it to turn grey?
theapocilip (1 year ago)
Wait a minute. In another video you said humans are the only animal who's hair will never stop growing on its own and that's why people can have extreamly long hair and animals can't.
rohit kumar (1 year ago)
8 months now my hair is growing
The Illusion (1 year ago)
why does everyones hair to grow at different rates and how do you gets yours to grow longer?
JungkBun (1 year ago)
My eyebrows grow very fast like no joke
Kaisli Rajamäki (1 year ago)
Why does hair change color from time to time? A hair strand can have different color through out its length.
Juubes (1 year ago)
Oh no little it doesn't hurt at all! Your hair is dead. WHAAAAAT I GOT DEAD HAIR IN MY HEADDDD?!!!! GET IT OOOOOUUUT!
A0nZ GoldY (1 year ago)
1.25cm a month?? Thats it! My hair grows 1/2 an inch every week and a half
babygirl (1 year ago)
My eyebrows grow faster than my hair... -_-
gentlebrit (1 year ago)
Still don't understand how the hair on my legs knows it's been shaven for it to start growing straight away, the hair didn't fall out naturally so how does it know? I get the part it knows when to stop.
Ernesto Estrada (1 year ago)
My hair grows around an inch each month.
Puneet shailly (1 year ago)
shave has maken my hair thicker
sun j shrestha (1 year ago)
i have hair on my lower part of my body how to control it
Brandon Rankin (1 year ago)
you say 1.25 cm a month meanwhile I'm a dude who's hair goes from Practically bald to below his shoulders in a month so I'm not so sure that fact is true XD
R Shannon (1 year ago)
1.25 cm a month?! my hair doesn't even grow till my bra strap... HOW can I make my anagen faze longer???
Cassie Ang (1 year ago)
wait my eyebrow NEVER drops.......
html1998 (1 year ago)
"the visible part of the hair is entirely dead"... so i wonder how they get "damaged" when bleaching or when you dye it
BKaten (1 year ago)
lol pause at 2:32
gonger03 (1 year ago)
why do kommondor dogs grow hair like a mop?
2Awesome (1 year ago)
What about the random white 8" eyebrow hair?
Saad Jirrari (1 year ago)
@SciShow so if my hair want 2 grow he need oxygene is that right beacause i put hats so if he want 2 grown up more i have 2 take off my hats and let him take oxygène
Darlene Rocha (1 year ago)
I am try grow my eyebrow long time
Crissy Square-Pants (1 year ago)
You did not answer the question tho... u just pointed out how weird it is
Bellona Eris (1 year ago)
Luis Lopez (1 year ago)
Funny. I'm watching this video and I get a hair treatment commercial.
Crystal Khlaire (1 year ago)
Ive never seen my skin hair fall out... never
Jognt (6 months ago)
You don't see the millions of skin cells falling off your skin on a daily basis either. Still happens though.

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