Mayonnaise initiates a scientific phenomenon called the Maillard reaction, which transforms not only the surface of an ingredient but also its chemical makeup.

When you grill bread coated in mayo, the amino acids and simple sugars in its structure get chemically altered, leaving you with a crispy finish that's more than skin-deep.

Since most mayonnaise is made with lemon or some other acid, swapping it out for butter can also add a subtle tang to the flavor of your grilled cheese.

Butter, on the other hand, acts as a moisture barrier, keeping your bread crunchy and crispy long after it's left the griddle — perfect for a packed lunch.

Butter may also be the better choice if you are using cheaper cheese slices, as its uniquely creamy and distinctly familiar flavor can also help mask the cheese’s artificial taste.