system
Given the following post, respond as if you're commenting on Reddit:
human
How do our hairs know when to stop growing?

How come my eyebrows for example stop at a certain length, yet if I cut them in half they'll grow back to that same size? How do they know their own length?
gpt
Hair does not know how long they have to grow. 

But they have a finite lifetime. After that they stop growing and they fall out. Also new hair is allways growing. The equilibrium is the terminal length where you think hair is no longer growing.

Bodyhair has a much lower liftetime than head hair, thats why eyebrowns are shorter.

If you don't cut your hair it will also reach a terminal length and it will look like its not growing anymore.