There's an old story about a physics professor who's writing equations on the chalkboard before a class.

"Now, it is obvious that Equation 12 follows from Equation 11, and from that we get Equation 13 ..."

Then he stops and stares at the board.  Erases Equation 12. Frowns, and writes it back again. He flips through a book, searching for something. Whips out a slide rule and fiddles with it. Scratches his head. Another erasure. More lookup. More computation. Another rewrite. And again. And again ...

After maybe half an hour he is satisfied. "I was correct: it is obvious!"