3 doors, 2 goats, 1 carhttp://www.youtube.com/watch?v=mhlc7peGlGg
You find yourself on a game show, you're given the choice of three doors. Behind one door is a car, behind the others, goats. You pick a door #1, and the host, who knows what's behind the doors, opens another door, say #3, which has a goat. He says to you, "Do you want to pick door #2?" Is it to your advantage to switch your choice of doors?
I still think the answer given is stupid as hell, even though I vaguely remember one of my maths teachers proving it to me. There're 2 doors to choose from, not 3, the chance can't be 33% or 66%.