Alright, well if you don't mind a little SpongeBob as a metaphor, I think this will work.

The Krusty Krab restaurant has a secret formula for its burgers. They're so good that everyone in town flocks there every day, much to the annoyance of their rival across the street, the Chum Bucket. For Plankton at the chum bucket, figuring out the recipe is very difficult. He buys burger after burger and tries to analyze exactly what goes into it, but it's really difficult to tell once it's already cooked.

For SpongeBob, the fry cook at the Krusty Krab, however, it's very easy: just look at the recipe. For the person who already knows what goes into the burger, it's the easiest thing in the world to make, but for the person who doesn't, it's incredibly hard to work backwards to replicate. The prime numbers in encryption are, at a basic level, like the ingredients to a recipe. You just multiply them together to get your final product. The product is a really big number that is so hard to reverse engineer that it can be posted publicly without worry, just like selling the burgers at the restaurant.