Bill Cosby tells the story about the guy who was mugged a number of times and decided he was going to take up Karate. He takes classes for about two years and gets a few belts (what ever the colors are)
He decides he's going to walk down the streets in the neighborhoods where he'd been mugged. Sure enough, some guy pops out of the ally and says 'gimme all your money" The victim jumps up in the air, throw a few "air" punches and yell KARATE! The mugger, unphased, swinging from behind his back yells .... MONKEYWRENCH!