Post 15- How Does it Know to Stop?

Do you ever notice that your fingernails never grow past a certain point? No matter how long you may give it, it just does not pass that very point. It's very annoying when you actually do want to grow your nails longer. Also there's your hair too, your leg hair, forearm hair, and just average skin hair, if you notice, if you don't shave these areas you'll see they don't grow past a point, sure I am grateful for this when it comes to body hair but I can't help but wonder why this happens.