Many people don't know how to recycle milk cartons, as evidenced by this photo of three milk cartons erroneously placed in the paper bin at my office.
Gus Lubin
Don't do this.
Here's the truth, taken straight from the Carton Council (the main organization for carton manufacturers in the US):
Cartons are primarily made of paper but also have a thin layer of polyethylene (plastic). Shelf-stable cartons contain a layer of aluminum. As such, milk cartons should be recycled with plastic, metal, and glass containers. Ultimately, the cartons will be sorted separately at the recycling facility and disposed of properly.
(The NYC Department of Sanitation gives the same instructions.)
A few other surprising facts to keep in mind:
You don't need to rinse out cartons before recycling. (Of course, you might want to if worried about smells.)
You also should not flatten the carton. Sorters at recycling facilities will have an easier time identifying the object if it's in its standard form.