I took an applied mathematics class once. I was too cheap to buy the class text, and instead used a free-to-use textbook written by an older alumni. Why this seemingly unrelated exposition, you ask? Hidden in the appendix of his free textbook was this gem:
"There are two important concepts in economics. The first is “Buy low, sell high”, which is self-explanitory. The second is opportunity cost, the highest valued alternative that must be sacrificed to attain something or otherwise
satisfy a want. I discovered this concept as an undergraduate[...]. I was never very in to computer games, but I found myself randomly playing tetris. Out of the blue I was struck by a revelation: 'I could be having sex right now.' I haven’t played a computer game since."