system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How come eyebrow, eyelash, arm hairs, ect only grow to a certain point, but when you shave them they grow back? How does it know it's been shaved? Why don't they continuously grow like head hair?
gpt
Hair length is dependent on how long each hair lives in a certain area. On your head hair can live for years before it dies and eventually falls out. Arm and leg hair might only last a month or so, so it stays short.

tl;dr
Your hair is always growing, it is how often it falls out that makes it stay short, or allows it to become long.