Chad Mulligan's favorite story/joke
A professor in [pick-a-subject] enters the classroom. As the students settle down for the beginning of the class, he puts up a number of propositions written in a dense symbolic notation. He then turns to the class and begins the lecture with, "I'm sure we can all assume that this" ... gesturing towards the displayed text ... "is obvious, can't we?"
He pauses briefly for effect before continuing. But then the pause becomes an unexpected interruption as he turns and just stares at the text. When he notices a colleague walking past the classroom's open door, he suddenly rushes out and grabs her by the arm. An extremely animated discussion follows.
As the minutes drag on with no sign that their discussion will ever end, the murmuring among the puzzled students grows louder and louder. Then the room suddenly falls silent again as the professor, his confidence restored, strides back into the classroom.
"I was right!", he informs the class. "It is obvious."