Will you change it or not?
There are 3 doors, behind which are 2 goats and a car.
You pick a door (call it door A). You're hoping for a car of course. The game show host, examines the other doors (B and C) and always opens one of them with a goat (Both doors might have goats; he'll randomly pick one to open).
Here's the game: Do you stick with door A (original guess) or switch to the unopened door?