Everyone has a butt, so you've probably seen one before -- maybe even your own. These two, rounded portions of pelvic fat hang out on either side of the anus.
We sit on our buttocks. We spread them when we need to poop. That covers the basics, but we also obsesses about them , reacting even to such an innocuous image as the above with worship or outrage.
So why do we have buttocks? Well, a lot of it just comes down to the sort of lower back that a bipedal ape winds up with over the course of evolution. The underlying muscles are vital to human movement, and buttocks allow primates to sit upright and take the weight off their feet.
Those are the basics, yet there's much more to learn. Let's see what else science can tell us about the butt and humanity's endless fascination with its own rear end.