https://en.wikipedia.org/wiki/Monty_Hall_problem
Consider a different problem first: you’re faced with the same three boxes but now you can choose any one box OR any two boxes, and in the latter case receive the better of the two contents. It’s always better to choose two boxes, right? But the rules of the original game allow you to choose two! Here’s how. First point to the remaining box i.e. the one you’re not going to choose. Then Monty will open the worse of the two boxes you chose, and you take the better one.