Vitamin D

First things first: it's not a vitamin. As with estrogen or testosterone, your body makes enough of it from available cholesterol, under the right conditions. If enough ultraviolet <radiation> - traditionally obtained from sunshine - gets through the tough outer layer of your skin, most will have all the Vitamin D they need to stay healthy.

Two questions naturally arise from this:
    1) How does Vitamin D keep you healthy?
    2) How do you make sure you're getting enough?