A "Cheese Grater" is someone of incredibly low inteligence. The term Cheese Grater comes from the belief that all inteligent people start with a base I.Q. Like cheese (you start with so much), and these imbeciles come along and shred every precious point from you; simmilarly to how one grates a cheese block.
Teacher: Melissa, explain to me the importance of Planck's Constant.