Can't Tickle Yourself? That's a Good Thing

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.