system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the hair on your head continue to grow while hair on your arms reaches a max length and no longer grows?
gpt
All hairs on your body go through cycles of growing and then falling out.  The hair on your head happens to have a longer cycle than other parts of your body.  Depending on the exact length of your cycle, everyone has a maximum length their hair will grow to.  Also, each individual hair is on a slightly different timing, so they don't all fall out at once.  That's why you never really notice it.  You're constantly losing and regrowing hairs, but it happens so smoothly that it looks like your hair has just stopped growing.