An emulsion is a mixture of two liquids that normally don't want to stay mixed together, like oil and water. If you've ever shaken up a bottle of salad dressing and watched it slowly separate back into layers, you've seen an emulsion form and then break apart.

The trick to making a lasting emulsion is adding a third ingredient called an emulsifier that helps the two liquids stay blended. Egg yolks, for instance, contain natural emulsifiers, which is why mayonnaise (a mixture of oil, lemon juice, and egg yolk) stays creamy instead of separating. Milk is also an emulsion: tiny droplets of fat stay suspended in water because of natural emulsifiers in milk.

Emulsions show up everywhere in daily life. Lotions and creams are emulsions that let water and oils blend into something smooth you can rub on your skin. Paint is an emulsion that keeps pigments evenly distributed in liquid. Even some medicines are emulsions, designed to mix ingredients that would otherwise separate.

When an emulsion breaks or separates, you'll see distinct layers form, like oil floating on top of vinegar.