Ask HN: Talks, tutorials, courses that made a programming topic click for you?

Sometimes it happens that on first encounter with a concept we are not able to fully grasp it. It remains mysterious until we encounter one single tutorial or lecture or an in-depth course on that topic sometime later. I never quite understood Python decorators until I read How to Design Programs book and understood higher order functions properly. Then while going through Berkeley's CS61A lectures by John DeNero I got them finally. Do you have any such incident in mind where something finally clicked from a talk, tutorial, summer school, course or whatever? The topic can be anything that can be broadly classified under programming or computer science.