Many people assume that pizza boxes are recyclable. In fact, most boxes have recycling symbols on them and are traditionally made from corrugated cardboard. They are, in and of themselves, recyclable.
However, what makes parts of them non-recyclable is the hot, tasty treat that comes inside them, specifically, the grease and cheese from pizza that soil the cardboard.
So there you have it, pizza boxes that are tarnished with food, or any paper product that is stained with grease or food, are not recyclable - unless you remove the tainted portions.
But why is this? And what are the implications for the general, pizza-loving public? Mmm, pizza. Read.