Ask HN: What elegant concepts (like recursion) should others be exposed to?

I'm studying state machines right now and I find the model for describing systems of change very interesting. State machines remind me of the _smell_ of recursion. They are are very powerful, yet rather simplistic in their most fundamental form, but have deep properties and far reaching application in problem solving. What are some other constructs that others may not be aware of (they don't need to be mathematical or specifically related to problem solving for that matter) that you find interesting?