Let’s say that you find yourself in the following situation. You know (don’t worry about how you know) that something that you find valuable, say an expensive ring, is in one of two cans but that the other one is empty. But you don’t know which can contains the ring. You also know that the person who has the cans will give you the ring if, but only if, you correctly choose the right can on the first try. Since the possessor of the cans really is a very nice person, he shakes one of the cans in your presence. And then, regardless of the outcome of the shaking, you know which can to choose so that you get the ring. How do you know?