The key to a perfectly gooey, delicious grilled cheese sandwich is, of course, the cheese. Some cheeses melt better than others, due to the interactions between the casein proteins and the calcium, while some cheeses will not melt at all. Gruyre, mozzarella, muenster, fontina, monterey jack, gouda, colby, and swiss varieties are excellent melting cheeses. Mild or medium cheddar melts evenly, but aged cheddar is more acidic, so it tends to separate. Processed cheeses, such as the classic "American, " Melt nicely because of the way they are developed. Some fresh cheeses, such as cream cheese, soft goat cheese, and cottage cheese, are best paired with a firmer cheese that offers a more robust flavor. Pair cream cheese or boursin with cheddar or mozzarella, or combine goat cheese or crumbled blue cheese with parmesan and cheddar. The bread is just as important as the cheese and there are just as many choices. Artisanal breads make exceptional grilled cheese sandwiches, or you might choose a basic white, whole wheat, thick slices of a seeded bread, pumpernickel, or rye loaf bread. A sweeter bread is another good alternative, but keep in mind that it will brown more quickly. A grilled cheese sandwich doesn't have to be confined to bread, cheese, and butter. Try bacon drippings, olive oil, or mayonnaise instead of butter. Adding sour pickles, crunchy potato chips, sharp mustard or spreading a bit of tangy-sweet onion jam on the bread before you add the cheese might be a pleasant surprise Take a look at the tips and recipe variations for more combinations and flavor ideas.