How to explain zero-knowledge protocols to your children (1998) [pdf]
My favorite example of this from my college encryption class is: let's say that there's a giant jar of jelly beans, and I tell you that my super power is that I can count how many jelly beans are in that jar. I don't want to tell you how many there are, and you might not even believe me if I did, so here is the test we'll run:I'll turn around, and you grab a handful of jelly beans and put them behind your back. I'll then turn back around, count the number of jelly beans in the jar, and tell you how many are in your hand.After repeating this 100 times, I will have demonstrated that I can count the number of jelly beans in the jar without telling you how many are in it.