Why does hair grow back?

My 10 year old asked me this question and I cannot find the answer.  Why does hair grow back after it is cut?  We understand that hair on your head, if not cut, will continue to grow and grow - so it makes sense to us that hair on our heads will grow back after it is cut.  However, other body hair seems to grow to a predetermined length and then stop (i.e. leg and arm hair, underarm and pubic hair, eyebrows).  So, why do these types of body hair grow back after shaving, or even after trimming with scissors?  How does the body know that the hair is shorter and why does it care?  I know this is not a pressing question, but I thank you for your time.