we're all geniuses in ap physics

while going over homework in class:

homework question: what is the centripetal force that keeps the earth moving around the sun?
brandon parks: god!
class: [lighthearted laughter]

btw, dictionary.com says that the word "geniuses" should be used to describe 2,3, or 8 geniuses, but "genii" describes 6,7, or 9 geniuses. who the hell comes up with these things?