Einstein and his driver

The story is that Albert Einstein's driver used to sit at the back of the hall during each of his lectures, and after a period of time, remarked to AE that he could probably give the lecture himself, haveing heard it several times. So at the next stop on the tour, Albert Einstein & the driver switched places, with Albert Einstein sitting at the back, in driver's uniform. The driver gave the lecture, flawlessly. At the end, a member of the audience asked a detailed question about some of the subject matter, upon which the lecturer replied, 'well, the answer to that question is quite simple, I bet that my driver, sitting up at the back, there, could answer it...'.