While I was at the municipal swimming pool today, I noticed something interesting. I was hanging out in the diving well with a few of my friends, sitting around and trying to stay afloat. The problem was that I kept sinking into the water. In fact, if I didn't vigorously kick my legs and flail my arms I slowly sank. While all of my other friends could lay on their backs and float, I was left bobbing upright with my head half an inch below the surface. Why is that? Why do certain people float and certain people sink?