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.
“It wasn't success, because Teller and I, by the time Asparagus Valley got together - within a year, we had achieved all our goals. I mean, our goal was to earn our living doing exactly what we wanted. Which is many people's goal.”