A bunch of college students miss a test. One, who knows the professor, tells him that they had a flat tire, and asks if they could retake the test. The profeessor says yes, on the condition that they take it in separate rooms. Flushed with their success, they are taking it, and they all look in horror as they flip it over and it says, "which tire?"
Three guys walk into a bar. Don't you think that the third one would have seen it?