Got it from Making Light:
Werner Heisenberg, Kurt Gödel, and Noam Chomsky walk into a bar. Heisenberg says, "It’s very odd and improbable that we three are in this bar together. It suggests to me that we’re in a joke, but I can’t be certain."I giggled for five minutes. But then, I'm a freak.
Gödel says, "Well, if we were outside the joke we would know, but since we’re inside it, there’s no way we can make that determination."
And Chomsky says, "Of course this is a joke, but you’re telling it wrong!"