Socrates: Define, for me, a punch line.
Hippias: A punch line is at the end of a joke.
Socrates: Is it a punch line simply by virtue of being at the end of said joke?
Hippias: No, it must be an unexpected statement.
Socrates: Ah, but if you know that the punch line is about to arrive, how can it be unexpected?
Hippias: True. Therefore, there can be no punch line to any joke, for such a punch line is always to be expected.
Socrates: Exactly. Last night the exact same logical conclusion was told to me by your mother, while we had intercourse.
1: Knock, knock.
2: Who’s there?
1: Just a statue of a horse.
2: Just a statue of a horse who?
1: Seriously, let us in.
Lysander, Xerxes, and Alexander the Great all walk up the Sphinx. The Sphinx says “I will ask you a riddle, and if you answer it, you may pass.” Lysander tries to answer it, but fails. He is devoured by the Sphinx. Xerxes tries to answer it as well, and also fails. He too is devoured. But when the Sphinx asks Alexander the Great the riddle, Alexander ponders the riddle for a brief moment and then chops the Sphinx in two. This joke shows that many seemingly intractable problems can be solved in novel ways.