If you have a cat, it's likely that at some point you've seen it do this: while playing with your foot it suddenly drops to its side, latches on with its front claws and begins a coordinated kicking motion with both back legs. The cat probably gave you a good four or five swats before letting go. From a human point of view this looks like great fun - a real rough-and-tumble kind of game. But where does it come from?

Before giving an answer, let's think for a second about the kind of thing we can learn even from so commonplace an event. House pets are not toys; although they are domesticated and, in the instance of cats, much smaller than their wild counterparts, it's a mistake to regard them as fundamentally different from wild animals. They share millions of years of evolutionary history, and their behaviors - even playful ones - are in many cases behavior that was very useful in the wild.

So, think again about that front-claw grab connected with a back-claw kick that your cat does from time to time. You may see it in a new light when you realize that the cat is executing a very effective device for killing prey or enemies: hold the other animal's stomach in place while you disembowel it.

Of course, that doesn't mean your cat is really trying to hurt you; but it does show us that playful behavior in animals -and, oftentimes, in humans - is not simply nonsense. In many cases it is a practice run for learning more useful skills - even deadly ones.