system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why does hair know when to stop growing

Can anyone please explain why the hair above my neck (eyebrows excluded) keeps on growing but all the rest grows to fixed lengths.
gpt
Hair growth happens in a 3-step cycle - the anagen phase, catagen phase, and telogen phase. The **anagen** phase is the growth phase, the **catagen** phase is the 'stopping' phase - where your hair stops growing and the hair follicle shrinks, and the **telogen** phase is the resting phase. At the beginning of a new anagen phase, the old hair is pushed out (what we call "shedding"), and the new hair grows in its place.

Different hairs grow to different lengths because the anagen phase lasts for different lengths of time on different parts of the body. It's also the reason why some peoples' hair grows very quickly, and other peoples' hair grows very slowly. Where, on your hear, the anagen phase can last for up to 6 years, on your arms or eyebrows the anagen phase will only last a few months.