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 just seems to be a human trait to want to protect the speech of people with whom we agree. For the First Amendment, that is not good enough. So it is really important that we protect First Amendment rights of people no matter what side of the line they are on.”
“Failure after long perseverance is much grander than never to have a striving good enough to be called a failure.”