Ask HN: What concept opened your mind to a new world?

I recently learned about two things that really opened my mind up to a new way of thinking about things.

1) learning about radio communications (including the Fast Fourier transform) and how all wireless communications use modulated radio waves to send information and

2) taking the Nand2Tetris course and learning about the basic components of a CPU working together to execute lines of code.

Both of these just really made me understand things differently. I’d love to know what other topics (math, computer science, electronics, anything) people found to be eye opening and changed their perspective.