Switch or Stay?

The Monty Hall Problem

There are three doors. One hides a car, the other two hide goats. You pick a door. Then the host, who knows what's behind each one, opens a different door to show you a goat.

Now you get to decide: keep your original door, or switch to the other one still closed?

Your first pick has a 1-in-3 chance of hiding the car, and that never changes. But the host's reveal doesn't spread that remaining 2-in-3 chance evenly. It collapses entirely onto the one door you didn't pick and the host didn't open. That's why switching wins about two-thirds of the time, while staying wins only about one-third.

Read more on Wikipedia.