The kind of cheese you choose matters, too. The proteins in an aged one will have broken down a bit, which makes them better able to emulsify in the sauce. (In the case of Velveeta, emulsifying salts are added to make the proteins more soluble.) If you bake your mac and cheese, another process takes place: the Maillard Reaction, in which amino acids and sugars react in the presence of heat to produce toasty, nutty flavors. "Macaroni and cheese has an ideal mix of compounds present for the Maillard Reaction," says Lloyd Metzger, Alfred Chair in Dairy Education at South Dakota State University. "And when the macaroni and cheese is heated a second time, a new group of flavor compounds are created." What's more, the gelatinization you initiated in the macaroni the first time around will have, over a day in the fridge, given way to retrogradation, in which the starch molecules align and form hydrogen bonds, giving the pasta an enhanced, toothsome firmness. Thus macaroni and cheese's capacity to taste even better, and more complex, the next day is built into the ingenious design of the dish. Fresh or leftover, it really is the perfect food.