Whether you’re a gouda groupie or a gruyère kinda guy, all grilled cheese sandwich enthusiasts can agree: The gooier the better, when it comes to this student staple.

In this video by the American Chemistry Society’s YouTube channel Reactions, they detail the precise science behind cheese and its gloriously gloopy properties.

That beautiful gooey mess is all to do with the acidity and calcium found in the cheese. The cheese’s casein proteins are collected in fatty spheres called micelles, which are held together by calcium. These micelles are repelled by each other, as they all have the same outer charge.

If cheese is left to mature, it becomes sharper and more acidic, as more of the sugar lactose is converted into lactic acid. The more acidic the cheese, the less the calcium molecules hold together the micelles, allowing all the proteins to break away, mingle and flow together.

However, if the pH levels are too low and acidic, the cheese will release all its oils when heated, causing a greasy mess.

So which cheeses hit the pH levels sweet spot? Apparently, it’s manchego, gruyère and gouda which have a pH level of 5.3 – 5.5. Those “luminescent” yellowy-orange slices of American processed cheese will also do a pretty good job, too. They’re usually made of different types of cheese and lots of emulsifiers and phosphate salts, which also limit the amount of calcium holding it all together.