I like this.

Suppose you are on a game show in which you can win a car, and you are given the choice of three doors - behind one door is a car, the others, nothing.

You pick a door, say no.1. The gameshow host, knowing what is behind the doors, then shows you one of the other doors (say no.3) which has nothing behind it.

The gameshow host then asks you if you would like to change your choice to door number 2. Is it to your advantage to switch?