system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it that hair grows to extensive lengths on your head, while leg and arm hairs remain quite short?
gpt
Hair doesn't actually stop growing. The different lengths you see get are just a result of them falling out before they can get any longer.

A follicle will produce new cells for a certain period of time depending on where it is located on your body. This period is called the growth phase called anogen. Then it will stop for a period of time (the rest phase called telogen), and then restart the growth phase again.

When the hair follicle enters the rest phase, the hair shaft breaks, so the existing hair falls out and a new hair takes its place. Therefore, the length of time that the hair is able to spend growing during the growth phase controls the maximum length of the hair. Scientists have observed hairs on different parts of the body. They find these ranges of growth for different kinds of hair. 

Scalp hair (Anagen , grows 4 to 8 years) (Telogen, rests 2 to 4 months)

Eyelashes. trunk, and extremities (Anagen, grows 1 to 6 months) (Telogen, rests 2 to 4 months)

Hope that helps!