For me it was lambda calculus. I remember in high-school going through the motions of beta-reduction but having no idea what it meant. Much later I think I fina

For me it was lambda calculus. I remember in high-school going through the motions of beta-reduction but having no idea what it meant. Much later I think I finally saw an implementation of natural numbers and addition using bare lambda calculus and it clicked that you really can represent any computation with just variables, abstraction, and application.