I didn't know that. I thought girls did it too. Of course, this now gives me an added excuse to go around staring at girls' legs. I think it's just some kind of nervous habit, really. It's like biting your nails.
Some people jiggle their leg.
I don't think it's possible to do both things at the same time though. It's like patting your head and rubbing your stomach. I'm sure I know you. Still, I don't know many cartoon caterpillar tamers. 2 books of a sexual nature.