Monty Hall is a famous paradox from a reader's letter to "Ask Marilyn" column in Parade magazine in 1990.

The problem is defined as: suppose you're on a game show, and you're given the choice of three doors: Behind one door is a car; behind the others, goats. You pick a door, say No. 1, and the host, who knows what's behind the doors, opens another door, say No. 3, which has a goat. He then says to you, "Do you want to pick door No. 2?" Is it to your advantage to switch your choice?

While the instinctive reaction is usually to think that the probabilities of either door are the same, the conditional probabilities that the car is behind door 1 and door 2 are 1/3 and 2/3 respectively. Thus the game show participant should always switch their choice, given the option.