I went to my Macroeconomics class tonight, and the teacher had a slide that showed the price of a Big Mac: "1940, $0.30 / 1997, $1.97". I piped up: "... except that McDonald's didn't exist until 1955, fifteen years later." A guy a few seats in front of me turned around and exclaimed, "Duuude! How did you KNOW that?"
How do you answer a question like that? "Dude, it's just knowledge."