There's a car behind one door, and goats behind the other two. When you choose a door, the host, Monty, will open a different one, and you can choose to stick with your original choice or switch to the other unopened door.
If the host knows where the car is, he will always reveal a goat - and you're more likely to find the car if you switch. However, if Monty doesn't know where the car is and opens a random door, there's no benefit to switching - even if he revealed a goat! Don't believe it? Try it for yourself!
In this version, Monty always reveals a goat behind one of the doors you didn't choose.
Choose a door!