Boys will be boys...I guess

The other night Adam and I were watching Sports Soup and there was a clip of a couple of teenage boys playing ping pong that quickly turned into a game of "sting pong," as they called it. One boy would raise the back of his shirt and stand with his back to the table while the other boy tried to hit the ball as hard as he could at the first boy's bare back. After the initial wince of pain, the two boys would laugh and laugh. Then they would switch places and laugh some more.

While we were watching this clip, I found it slightly amusing, but Adam was laughing hysterically. He explained to me that this is what boys do, hurt each other and laugh at it. I don't really understand it, but Adam grew up with 5 brothers, so I guess he knows what he's talking about. He also has the scars to prove it.

Can anyone else explain why boys do what they do, or is this just one of the unexplainable differences between boys and girls?