system
Given the following post, respond as if you're commenting on Reddit:
human
Why does the hair on our body seem to stop at a certain length while the hair on our heads grow continuously?
gpt
All hair grows for a certain amount of time, stops for a period, then falls out.  The cycle repeats.  Changing the amount of grow and dormant times gives you the difference between scalp hair and arm hair.  This basically gives all hair a maximum length and the hair that remains a constant length has a short enough growth period to show you that average length. That and the fact that you don't cut it.