He didn’t mention it in this letter, but in Surely You’re Joking Feynman described how he solved for the motion of plates spinning and wobbling across a cafeter

He didn’t mention it in this letter, but in Surely You’re Joking Feynman described how he solved for the motion of plates spinning and wobbling across a cafeteria in Cornell. When his colleague asked what’s the importance of this problem, he responded: “There’s no importance whatsoever. I’m just doing it for the fun of it.”