This happened in a class I was attending a few years ago. While giving a lecture on the mechanics of momentum and kinetic energy transfer, my physics professor noticed a student busily snoozing in the fourth row of the auditorium. He turned to a girl sitting in the first row and asked her what the result of a collision between two particles with masses, velocities, and trajectories that he specified would be. After a few seconds of thinking and calculating she came up with the correct answer. The professor then awoke the sleeping student and said, "You! Same question!" The student, a bit stunned and bleary eyed, looked around and replied, "Same answer." After the laughter died down the professor turned around and said, "OK, you got me on that one, I won't call on you again..."
(From the "Rest" of RHF)