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Monty Hall Paradox

This puzzle -- introduced to me by New Scientist -- is truly wonderful in its capacity for making fools of intelligent people.

The idea is simple. The setting is a gameshow, and you're trying to win the grand prize. You are presented with three doors. Behind one is the prize, behind the others, nothing.

You pick a door at random, since there is no difference between the three. The host, who of course knows where the prize is, now opens one of the doors you didn't pick and shows that no prize is behind it.

You're now given the choice to stick with your original choice or switch to the remaining unopened door. What will you do?

The Answer

Actually I'm not going to give the answer -- since it's worth thinking about this for a while if you haven't heard it before. Once you're absolutely sure you have the answer, Google will tell you whether you got it right. But will you believe it?

As I said, this puzzle is brilliant because even the most intelligent people get it wrong. If it was even remotely important it'd be the sort of thing wars get started over, because nobody wants to back down once they think they have the answer.

If by some chance you should settle on an answer that Google tells you is wrong, I suggest getting a pack of cards and trying it for yourself. You should see very soon what the right answer is.

And of course, once you know the answer, you can torment other people with it, secure in the knowledge that you're absolutely right. Have fun!

This puzzle goes hand in hand with the Envelopes Paradox.