When I think about things I find interesting in math, I keep circling back to 1. In some ways, it’s the simplest number—just one thing. And yet, 1 wears so many disguises. The invisible 1 trips students up all the time: in 3x + x, in x^1, in coefficients that are “there but not written.” When we simplify fractions, rationalize denominators, or even dividing by a complex number, we handle the problem by multiplying by one—just not the obvious one. We choose a form of 1 that changes everything while changing nothing I’m curious—what’s a concept like this for you? Something small or “obvious” that turns out to be doing a ton of hidden work in math?