The Monty Hall Problem
You are attempting to win a car. There are three doors in front of you, you are told that behind one is a car and behind the other two are goats. You are asked to pick a door, but before it is opened one of the two unchosen doors is opened to reveal a goat. You are then asked if you wish to swap your choice of door or stick with your original choice. What do you do to ensure you win the car?