2018-05-18
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, that is why eyebrows are shorter than the hair on head.
If you don't cut your hair it will also reach a terminal length and it will look like its not growing anymore.