I'm in university, but my favorite class this semester is the organic chemistry lab. Organic chemistry is infamously difficult, and I failed the lecture the first time I took it (and had to re-take it), but for some reason it began to grow on me after a while. Hard and stressful, but simultaneously enjoyable.
Back in high school during my senior year, I loved my advanced animation course. It was over an hour long. My class collaborated on a video game and I was the concept artist, so after I got my work done, I spent pretty much every single day just playing Minecraft or Skyrim, or using Tumblr and searching up dumb memes with my friends. It was such a laid back class where I got a lot of free time, and I'd do anything to go back to that time in my life, haha.
