A man and his son are driving home one day when they get into a terrible accident. The man dies on the way to the hospital, but the son can be saved by emergency surgery. The surgeon on duty takes one look at the boy and walks out of the theatre, saying to the nurses: "I can't operate. That's my son."This is an old chestnut, but effective. Some of us at COB, when teaching our students about logic, include it as a test of their critical thinking skills. When you know the trick, the answer is obvious, but otherwise it seems to be baffling. Again and again we find that students don't see through the problem, can't see what becomes obvious the moment you realize that the point from which you start to reason is fundamental to the solution of the puzzle.