Question: "Why are manhole covers round?"
Cognitive Science (MIT) answer: "They only appear round - they're actually cylindrical."
Mechanical Engineering (MIT) answer, year one: "Because that's the only cover shape that can't turn sideways and fall in." (nope - extra points for the counterexample proof).
Mechanical Engineering (MIT) answer, year two: "Hoop stress." (good guess, but still wrong).
Social Science (Stanford) answer: "Because all the other manhole covers are round."
Architecture (Berkeley Extension) answer: "Because manholes are round."