[mythas]

My preferred example: I want to prove to you I know how to do a card trick. I have you pick a card memorize it and put it in the deck. Then I am able to reproduce your card. I can do this a million times to convince you, but never reveal the method of the trick to you. As long as when the verifier picks a card they don't show it to anyone, any other spectators can't know if we are colluding and can't determine how the trick is done.

I prefer this example as it is easier to visual knowing how to do a card trick than it is to understand how you would identify the number of beans in a jar. It also allows you to further explain things like collisions as a magic trick can have multiple methods. Or how by observing enough cases of the trick and "thinking" really hard you might be able to figure out how the trick is done. The crappier the magician the easier it is to figure out their trick.