The setting is someplace like Vienna in the mid-19th century. One student asks his professor to explain how the ventilators on the tramcars work. The professor exclaims, "That's really easy!" and proceeds to provide a lengthy explanation. However the student responds, "But, Herr Professor, the vanes rotate in the other direction." Whereupon the professor retorts, "Ah - that's even easier to explain then."
Not sure where I read this - it might have been from Rhodes' book The Making of the Atomic Bomb which incidentally I consider the best non-fiction book I've ever read.