system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:  How can your body tell that your hair has grown to it's desired length?

Body hair, for example, stops growing once it hits the desired length.  How does each hair follicle know when to stop making new hair?
gpt
It's just controlled by growth time. So arm hair follicles crank out hair for say 2 weeks, stop, the hair falls out, they rest, then the cycle starts over with a new hair. Hair on your head has a longer hair-producing part of the cycle, leading to a longer hair.