Milk is classified as a colloid. A colloid is a mixture in which very small particles are spread evenly through another substance. Specifically, milk is a type of colloid called an emulsion. Emulsions consist of liquids spread through other liquids. The liquids in an emulsion do not completely mix like the particles in a solution do. Instead the particles of one liquid are suspended in the other. Emulsions generally have a cloudy appearance and will often separate if not continuously mixed. Milk is a natural emulsion of fats (oils) and proteins spread throughout water. Because milk is mainly made of water, it has properties similar to water, such as a high surface tension. Surface tension is a property of liquids that describes the attraction of liquid particles at the surface. Soap is a surfactant. A surfactant is a substance that has the ability to reduce the surface tension of a liquid. Therefore, when a drop of liquid dish soap is added to milk, the surface tension of the milk is reduced. As this occurs, the fat and protein particles in the milk can move more freely and easily. In addition, the molecules that make up soaps and detergents have two main parts (ends) that behave differently. The ends attracted to fat will move and clump together around the fat particles. As this movement occurs, the dye moves through the milk as well, creating colorful swirls in the milk.