Joke: Werner Heisenberg, Kurt Godel, and Noam Chomsky walk into a bar.
Heisenberg looks around and says "Because there are three of us and this is a bar, it must be a joke, but is it funny?"
Godel answers "We can't tell from inside the joke. We have to be outside looking at it to tell."
Chomsky looks at both of them and says "Of course it's funny. You're just telling it wrong."
Explanation of Joke:
Heisenberg: His "Uncertainty Principle" states that you cannot determine the position AND the velocity of an object at the same time.
Godel: Math systems cannot be proven from within themselves.
Chomsky: A famous linguist, and also apparently terribly opinionated.
More "___walked into a bar" jokes at a thread on Making Light.