Ask HN: What is the single most important concept to understand for programming?

Ask HN: What is the single most important concept to understand for programming?
6 by Nadya | 3 comments on Hacker News.
Or alternatively - what was the single concept that made you have an "Ah ha!" moment where everything you had read about and learned before fell into place? I ask because nearly a year ago - I finally had my "Ah ha!" moment. Now, while I always had understood arrays on a basic level, they never truly "clicked" with me and I often found it difficult to read from or manipulate multi-dimensional arrays. One day though - it clicked and everything else fell into place. My efficiency skyrocketed and I went from feeling like a script kiddie to feeling I actually finally understand what's going on. I'm going to be in charge of training some juniors at the company I work for. I believe good teaching should focus on the things that provide the greatest value for understanding - so I'm crowdsourcing other peoples' epiphanies in hopes of giving my juniors their own. :)