How does the hair on your arm know to stop growing at a certain length, but the hair on your head won't stop?

Question

How does the hair on your arm know to stop growing at a certain length, but the hair on your head won't stop?

Answer 1

Each person has a different limit to how long his or her hair can grow, but don't reach it since it is generally socially unacceptable. Most people can't grow hair much past their tushes...

Answer 2

Body hair is programmed to grow in cycles. Once they "rest" the shaft breaks and is replaced by a new hair. The stopping and starting is what keeps the hairs short as opposed to head hair which grows for much longer periods.