Flour tortillas, shredded mozzarella cheese, and pepperoni. (I've also used ham, mushrooms, and other pizza toppings.)
For ultimate meltiness, layer cheese and pepperoni.
Heat pan to medium-high. Place closed quesadilla on pan, let it warm on each side for 1-2 minutes. Let cool before serving to young children, cheese will be hot after cooking! Serve with warmed spaghetti sauce or cold ranch dressing for dipping!