What was that concept and what was the epiphany that made it all make sense?
One day, I was at a small party that had started to break up. The remaining guys were in one room, and the two girls had drifted to the kitchen and were chatting excitedly about something. I had gotten bored with what the guys were talking about, and wondered what the girls were so excited about. But I couldn't understand them because they were talking on top of each other. Then it hit me: They can understand each other! Women can talk and understand simultaneously, we can't. They can use the secondary areas in the non-dominant hemisphere as back-up language comprehension areas. Men just use areas in one hemisphere so we have to switch back and forth between speaking and listening, we can't do both at the same time. It's blatantly obvious if you stop to think about it, but in all the research i'd been reading, nobody had stopped to think about it.
The feminist take on language and gender made sense too. If you assume that men could listen and talk at the same time if they wanted to, comments that guys make, like "Hey, shut up a minuit would you, I want to say something" and "Hey wait a minuit, I'm still talking" do seem domineering.
Linear Algebra. The epiphany is difficult to explain and remember. It involved the elegance and coolness of eigenvectors.
How damned important and awesome physics is to life. The epiphany came after actually predicting precisely how a projectile would fall and where it would hit when launched. My only thought was "Oh my god, why didn't they tell me I could predict the future long before I got sucked into CompSci? I would be a physics major had they shown me this stuff earlier. Why isn't teaching better? How many other people never discover this and abandon interest?"
Why people tried hard at all in school. Epiphany: "Because people really do reward you for excellence after school. Even if doing well in school feels like you're being penalized and singled-out, all those other people will be slacking off after school while you're naturally being successful."