Puzzle 5: Candy Boxes

Allie and Matt stand in front of the classroom, and each one of them is holding a box. One box contains a massive pile of candy, and the other contains one very old piece of Jet's pizza.

One mentor always lies, and the other always tells the truth.

Each mentor knows the contents of each box, and they also know about the other mentor.

You are allowed to ask one yes or no question.

What question do you ask to figure out which box contains the candy?