The hardest class, in terms of failure rates, in first year CS in my university was Discrete Mathematics. I got an A- and was pretty proud of myself. In third

The hardest class, in terms of failure rates, in first year CS in my university was Discrete Mathematics. I got an A- and was pretty proud of myself. In third year, I became a TA for the course, marking assignments and hosting tutoring office hours. That's when I discovered that apparently I had learned nothing when I got that A-. Teaching a subject requires you to actually understand it, and not just know enough to pass the test.