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.
“Starting out, I bet I didn't get a lot of parts because of my strange voice. I'm not consciously thinking, 'Hey, sound like a squeaky dog toy mixed with a bagful of rusty nails.' It's just what my voice has done.”