Monty Hall Problem
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 other 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 take the switch?"
Yes it's in your interest because the host (Monty) will always choose a door with a goat behind it.