Foam is a mass of tiny bubbles clustered together, creating a light, airy substance that's part air and part liquid (or sometimes solid). When you squeeze dish soap into running water, foam forms on top: thousands of little bubbles trapped together in a frothy layer. The foam on top of root beer or hot chocolate comes from gas bubbles getting trapped in the liquid.

Foam appears everywhere in daily life. Ocean waves create sea foam when they crash and churn, mixing air into the water. Shaving cream is thick foam that makes razors glide smoothly. Fire extinguishers spray foam to smother flames by cutting off their oxygen supply.

Foam can also be solid. The squishy material inside couch cushions and mattresses is foam: air bubbles permanently trapped in rubber or plastic, making something soft yet supportive. Styrofoam (despite its name, actually a brand name for a type of polystyrene foam) helps keep your cocoa hot because all those tiny air pockets block heat from escaping.