This is why pizza boxes can't be recycled

Excerpts from an article at Earth911.com:

But when paper products, like cardboard, are recycled, they are mixed with water and turned into a slurry... Grease from pizza boxes causes oil to form at the top of the slurry, and paper fibers cannot separate from oils during the pulping process. Essentially, this contaminant causes the entire batch to be ruined. This is the reason that other food related items are non-recyclable (used paper plates, used napkins, used paper towels, etc).

“The oil gets in when you’re doing your process of making paper,” said Terry Gellenbeck, a solid waste administrative analyst for the City of Phoenix. “The oil causes great problems for the quality of the paper, especially the binding of the fibers. It puts in contaminants, so when they do squeeze the water out, it has spots and holes.”

Suggested solution for avid recyclers: Cut the empty box up, trashing the food-stained part, and recycling the top and clean parts.