In a TV game show, a contestant is shown three closed doors. Behind one of the doors is a shiny expensive sports car, but behind the other two are goats. The contestant chooses one of the doors at random (she has, after all, a one-in-three chance of winning the car). The game show host (who knows where the car is really located) flings open one of the other two doors to reveal a goat. He grins at the contestant and says: ``Well done, you didn't pick the goat behind this door.'' (Audience applauds sycophantically.) He then adds, still grinning: ``But do you want to swap and choose the other closed door or stick with your original choice?'' What should she do?