In Monty Hall's show (or rather, its version in the problem), a contestant would be given three doors to choose, with one having a prize and the other two not having anything. One of the doors not chosen by the contestant would be opened, to reveal it had nothing inside, meaning one of the two remaining doors has the prize. The contestant would then be asked if they'd like to change their pick - the problem, proposed in Marilyn vos Savant's column, is that if you do accept this offer and change your pick of the door, your chances of winning actually increase - a conclusion that was very hard to countenance for a lot of people.