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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 (812)
Eight Great (17 hours ago)
Why do i have hair growing on my neck in front my throat
Rhys Logan (3 days ago)
OK but if hair stops growing doesn't that make it fur?
Piano Girl (9 days ago)
I once accidentally shaved my eyebrows in 8th grade. Had to wear Sharpie for months lol.
Anastasija Filipova (17 days ago)
I'm 12 and my hair grows so long:(
Klaus Hoxha (23 days ago)
But there’s this woman that has super long hair can someone explain that to me
Rayan Ighil (24 days 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 (26 days ago)
Geekish Goddess (1 month ago)
Why is my leg hair curly like my hair
Carlos Ramirez (1 month ago)
"How does hair know to grow... Does it touch your brain?"
Noob Gaming101 (1 month 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 (1 month ago)
God don’t play 🙏🏾
LydiA Jkessi (1 month ago)
I think my hair stop growing when it reaches bottom of my chest is that possible
mos ab (2 months ago)
So, when a hair in catagen or telogen is cut, it won't grow back?
Batabii (2 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 (2 months ago)
Short answer: "We don't know" In my mind, what's the point of the video, then?
masterofthecontinuum (2 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 (2 months ago)
Here's another one: why do black people's hair grow slower than white people hair.
Loopy Poppy (2 months ago)
but why does my head hair stop growing at the point when it gets to my shoulders (am black)
Sean Peacock (2 months ago)
I have 2 individual eyebrow hairs that grow about twice as long as the rest. It is occasionally annoying.
Jimmy Joe (2 months ago)
I don't remember seeing my arm or leg hair fall out, only eyelashes, armpit/pubic and head hair.
Red Point (2 months ago)
So... No Rapunzel.... Allright...
TheRanchyestRancher24 (2 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 (2 months ago)
Evanston Conner (2 months ago)
I wish we can control hair groth by folicue
NWA362 (2 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 (2 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 (2 months ago)
does plucking stop hair growth ?
UNSC INFINITY (2 months ago)
What about my Ass Hairs?? ... It's like a freakin Amazon Jungle down there!!
RAHUL (2 months ago)
Hello hank Green
goodman854 (2 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 (3 months ago)
After a hair cut within three days you can tell my hair is growing back
DodgeMartin (3 months ago)
While I was watching this I couldn't stop looking at his hair
iZorohhh (3 months ago)
Well my eyebrows stopped growing for a while and one side of my eyebrow looks like it is shaved off...
Academic Abbreviatory (4 months ago)
I got a add for male pattern baldness for this video
darrick steele (4 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.
Qwerty uiop (4 months ago)
why does my hair grow faster in some areas?
Jognt (2 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 (4 months ago)
Leongon draws stuff (4 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.
madscientistshusta (5 months ago)
Dat close up!
Alexa Bella Muerte (5 months ago)
Rage Coder (5 months ago)
I thought they don't stop growing
adeQuate (6 months ago)
Then why do how do they know when to grow and how long when cut, it they are not connected?
SugarCofee (6 months ago)
''try explaining that to a 3yo'' hahahahahahhahahahahahhahahahahahahhaha
michael (6 months ago)
Why does women grow longer hair and more straight than men?
Jognt (2 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 (6 months ago)
1:01 Me at that age though
runnerJong (6 months ago)
why do some people grow pubic hair on their faces?
krishnaa balaji (6 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 (6 months ago)
Can you make a video on how hair loss serums and tablets work?
Mateolevic (7 months ago)
Intelligent design. Amen.
candlewax122 (7 months ago)
Okay, but how do I *stop* anagen?
Mike Altieri (7 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 (7 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 (8 months ago)
I'm only 12 years old but I had a very long hair on legs
Julia George (8 months ago)
😩i will start form know we will going to see what is going to happen 🤔
Farshad (8 months 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 (9 months ago)
Keratin-o-cytes. Because they hold large amounts of keratin protein. Not kera-atin-cyte....
qthoshi (9 months ago)
Why do older men start growing longer eyebrow hairs, nose hairs and hair in their ears?
UriYah Ysrayl (10 months ago)
Answer: God
Jay-Rock (10 months ago)
I want my hair back please giving me a solution without that Bosley surgery.
Here I'm (10 months ago)
How can we kill *keratinocytes* on our body without _laser treatment?_
ram Bhagat (10 months ago)
why fish haven't hairs😯
Amarilis (11 months 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 (11 months 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 (11 months 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
illusion yeet (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.
Nella Bella (1 year ago)
Ive never seen my skin hair fall out... never
Jognt (2 months ago)
You don't see the millions of skin cells falling off your skin on a daily basis either. Still happens though.
Akash Singh (1 year ago)
what is the reason of hair stop growing? is any genetic disorder?
thicboy (1 year ago)
this is a weird question lmao but does food coloring affect ur poop👀
Júlia Kercsmár (1 year ago)
my bday cake was blue and too big for the family for one night. :D trust me i was a unicorn for like a week. XD
Olter (1 year ago)
Chirag Manshani (1 year ago)
thnx it gives me nice experience
Michelle Tabisz (1 year ago)
Well that could have been, like, a five second video. Question: How does hair know when to stop growing? Answer: We don't know. But we do know is scientists want to know too! So you're just going to have to wait on that. Until then, watch this video to find out how hair grows and falls out!
Jognt (2 months ago)
That just means that you don't understand the video... Hair stops growing at the interval it is genetically programmed to do so (the cycle). Then that hair will be shed and replaced by a new hair.
MoochiAndLovebug (1 year ago)
Since we're on the subject id like to know sciences way to grow hair faster 🤔
Teresa Muriel (1 year ago)
I have always wondered this!
DreadlockDrummer (1 year ago)
why do men go bald, why on the top of the head, and why cant scientists figure out how to cure it?
Typhoon792 (1 year ago)
Was your hair intentionally even more fucked up than usual for this video?
I'm 15 but my armpits doesn't have hair in it yet. Is this like a genetic thing or does it come for me later in life?
Jognt (2 months ago)
It varies. For me beard and bodyhair growth picked up its pace in my twenties. I'm 31 and people often guess I'm 25 hehe. FeelsGoodMan
Olter (1 year ago)
+Yunas Guardian You dont HAVE to.
Yeager Comics (1 year ago)
Its gonna vary from person to person. I can't recall when mine started growing in. Genetics will play a roll. Give it time. I'm 28 and wish i could go back to not having to shave my face every morning :/
Jade Harris (1 year ago)
Or you could be like my brother and I and have one underarm grow more than the other.
BledarMartinaj (1 year ago)
It comes later im 15 and a half and it ha finally started to grow!
Meagan Mutuc (1 year ago)
this gives me a better understanding of how hair works thanks
Kueeny Kind (1 year ago)
Thanks SciShow!

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