I recently re-read a story about Max Planck from a transcript of a speech by Charlie Munger.  He describes it as “apocryphal”, and I have to agree.

After he won his nobel prize, he was invited to lecture everywhere, and he had this chauffeur who drove him to give public lectures all through Germany.  And the chauffeur memorized the lecture, and so one day he said, “Gee, Professor Planck, why don’t you let me try it by switching places?”  And so he went up and gave the lecture.  At the end of it, some physicist stood up and posed a question of extreme difficulty. But the chauffeur was up to it.  “Well,” he said, “I’m surprised that a citizen of an advanced city like Munich is asking so elementary a question, so I’m going to ask my chauffeur to respond.”

I guess the moral of the story is that you shouldn’t talk about what you don’t know — unless you’re as witty as the chauffeur.