system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do your eyebrows and eyelashes know when to stop growing?
gpt
They don’t.  All hair on your body has a “lifespan” where it continually grows hair at a set rate (with minor seasonal variation) and then “dies” and falls out.  The set lifespan of the follicle determines how long the hair can get.  The ones on the top of your head have the longest lifespans, but it varies among individuals.  Not everyone could grow waist-length hair if they even tried, for example.  You are constantly losing eyebrow hairs — they’re just so short that you never really notice them lying around.  People tend to notice eyelashes more when they fall out, because they sometimes end up in your eye instead.