I was randomly looking for quick and easy science projects online and saw one that takes no time at all. Just pour milk into a Coke and let it sit.
It is a reaction of the Phosphoric Acid contained in the coca cola to the milk. Phosphoric Acid molecules attach to the milk giving them more density and separate out while the remaining liquid that makes up the milk and cocoa cola now being lighter floats on top. The solid matter is basically milk that has been curdled by the addition of the more acidic soda.
Both items are acidic but Coca-Cola more so. In general, Coca-Cola has a pH of anywhere from 2.5 to 4.5 because of the phosphoric acid content whereas milk has a normal pH around 6.7 (almost neutral).