As a child, my brother would frequently challenge me to a game he called punch-for-punch. He'd let me hit him in the arm if he could hit me back just as hard. It wasn't a long game; dull punches soon became bruising wallops. Being several years younger and many pounds lighter, I'd often concede quickly, fearing the next blow that, despite the game's equally-hard rule, always felt more forceful than the last. The thing is, my brother and I were both playing by the rules--at least, we thought we were.

Some of us are more ticklish than others, but nearly everyone is unable to tickle themselves. The answer is tied to how we see and how we perceive movement. To get to the bottom of why we can't tickle ourselves, let's first examine another phenomenon. Close one eye, and then carefully push against the side of your other (open) eye, moving the eyeball from side to side in its socket. It should appear as if the world is moving, even though you know it isn't.