There are three doors. Behind one of them is a car. You want that car. Behind the other two doors are goats.
You pick a door, but it is not opened.
The host, Monty, opens one other door at random, and he asks you if you want to keep your selection, or switch to the third door, which has not been opened.
Monty has no idea what is behind each door, and nor do you. Is it better for you to switch your selection, or not?