system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does head/facial hair grow indefinitely but body hair only grows to a centimetre or two?
gpt
Each hair root will grow for a certain length of time, and then stop, shed its hair, and start over.  If the length of time it grows for is just a few weeks, then the hairs have time to grow very long, like the hairs on most people's body.  If it's a couple of months, the hairs have time to get a bit longer, like your eyebrows.  If the hair root will grow for several years, then the hair can become extremely long, like the hair on your head.  But even though the hair on your head becomes very long, it doesn't grow "indefinitely" - if you stop cutting it completely, it will eventually reach its (quite long) natural length, just like the other hairs on your body.