system
Given the following post, respond as if you're commenting on Reddit:
human
How does the hair on my legs know when to stop growing?

I have always wondered whenever I shave parts of my legs or light them on fire using my lighter, how they actually know when to stop growing into leg beards.

I tried sample groups like shaving a portion with razors and a portion of my legs with lighter.
gpt
The hair doesn't start growing when you cut it, and stop when it reaches its max length.  Instead, every single hair continuously grows for a set period of time, falls out, and the re-grows from scratch, over and over and over.

However, the follicles are not all synched up, so the hairs in an area are all at slightly different lengths.  If you look closely, you should be able to find quite a few short hairs (that you haven't cut recently, and are short because their predecessor recently fell out), but they're hard to spot with a quick glance, because they're mixed in with a lot of longer hairs.

So when you cut the hairs, the ones that were already in their growth phase will continue to grow.  Some of them were close to their max growth already, and will soon fall out without ever getting back to their max length.  Others had just started growing, and will continue to grow for much longer.  Over time, all the hairs will reach equilibrium.