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
http://earth911.com/news/2009/03/02/the-pizza-box-mystery/
