If there’s one A-to-Z essay I keep referring people to it’s the one about polynomials. So there’s no need to bring that up again. Instead I’ll pick up one from last year’s A-to-Z, about the Pigeonhole Principle. It’s one of those ideas so simple it hardly seems like anything. But it keeps implying weird and surprising and even counter-intuitive things. So that’s worth highlighting, particularly since some of the implications of the Pigeonhole Principle surprised me when I reread my own essay.