I just HAVE to add the old (and, unfortunately not true) story:
A college math professor had spent about half the class period writing a proof on the blackboard. At one point, as his writing a formula, he says "now it is obvious that", then steps back, looks at the formula and says "why is that obvious?"
He sits down, and writes furiously on a pad of paper. Just as the class is about to end, he jumps up and cries "Yes, it is obvious!".