A stable emulsion is a mixture of two liquids that do not normally mix, such as oil and water, that stays uniformly blended instead of separating into layers under the conditions it was designed for. Milk, mayonnaise, and vinaigrette-style dressings are everyday examples. Stability comes from emulsifiers — proteins, lecithin, or mono- and diglycerides — that sit at the oil-water interface and stop droplets coalescing, often helped by homogenisation, which breaks the droplets into very small, evenly sized spheres. Temperature, pH, salt level, and storage time all test stability: a dressing that is stable at room temperature may split in the freezer. Food scientists assess stability by watching for creaming, sedimentation, or oiling-off over time.