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?" Would it be beneficial for you to switch your door ?
Please do not check the answer from any other source. Think about it on your own. It yields a surprising result but is easy to solve.