A reader named Brooks wanted to know: "Why does the hair on your head constantly grow while the hair on your body has a limit?"
Technically, the hair on your head has a limit too -- like all hair, it has a growth spurt followed by a dormant phase. Here's an explanation from, of all places, a website about dogs:
Anagen is a period of new hair growth. The longer the anagen period, the longer the hair grows. Human scalp hair may stay in anagen for 2-6 years. Human hair on arms and legs may stay in anagen for only 30-45 days. Catagen is a transition phase. During catagen growth stops and the outer root sheath shrinks attaching to the root of the hair. Telogen is the resting phase. For human scalp hair the resting phase is about 100 days. The telogen phase for human hair on arms and legs is much longer than for scalp hair. Exogen is when the hair falls out, and the follicle enters a new anagen period, growing a new strand of hair.
So, in other words, your head hair grows for a long time but doesn't stay put for more than a couple of months after it's done -- right now, probably about 90 percent of your head hairs are having a growth spurt -- whereas your leg hair grows just a little and then clings on stubbornly for months without falling out.
For a deeper, more evolutionary explanation -- and some ruminations on the difference between "hair" and "fur" -- check out Science Daily.