system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do arm/leg hairs stop growing once they reach a certain length, yet other hairs continue to grow?
gpt
They don't stop once they reach a certain length.  They stop when they've been growing for a certain amount of time.  Hair follicles have a cycle where they'll grow for x number of days, rest for y number of days, then start again.  The length they grow to is just the length they can reach in that time.  After that, they stop and eventually fall out.  Then, new hairs grow and take their place.  The same happen on most of the body.  Really, it's only the hairs on the top of the head (and maybe the chin) that are different.