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Mixing milk with Coca-Cola creates a strange separation effect that may look alarming, but it's simply the result of basic chemistry. Coca-Cola is highly acidic, while milk contains proteins such as casein that normally remain evenly dispersed throughout the liquid. When the two are combined, the acidic environment causes these proteins to lose stability and clump together, forming solid particles that trap some of the soda's coloring. Because this material is denser than the remaining liquid, it gradually sinks to the bottom and creates the unusual sludge-like layer. Although the reaction may look unpleasant, it's a fascinating demonstration of how changes in acidity can alter the structure and behavior of proteins.