A joke is not a thing but a process, a trick you play on the listener's mind. You start him off toward a plausible goal, and then by a sudden twist you land him nowhere at all or just where he didn't expect to go.
“I actually think Bill Gates is conventionally smarter, even though it's a dumb word, but mental processing power - I've watched him use four different screens, process information, get to the right answer, boom boom boom.”