Originally Posted by abc
Possibly, but why does most body hair (all of it except for hair on your head, unless I'm forgetting something) grow to a certain length, then stop until you shave or cut it off?
As far as I can remember the answer to this is not that hair "knows when to stop growing" - it's just that it falls out after a certain amount of time. So head hair will keep on growing for a long time, while eyebrow hair grows for less time. Of course at any particular moment a particular patch of hair that has been left to grow (not clipped / trimmed) will have hairs of different lengths, but the overall impression is that the hair is an even length.