A microwave heats food by making water molecules inside it vibrate and warm their surroundings.

The longer version

Microwave ovens fill a sealed box with electromagnetic waves of a particular frequency. Water molecules carry a slight electrical imbalance, so the shifting field tugs them back and forth and makes them rotate faster. Their extra motion is heat, and it spreads to the rest of the food by ordinary contact.

The waves penetrate some distance into the material rather than only warming the outside, which is why thick items can heat unevenly. Fats and sugars also respond, often heating faster than water. Metal reflects the waves, and the pattern inside the box has fixed hot and cold spots, hence the rotating plate.

Where this entry sits

Everyday physics and chemistry hide small wonders in plain sight, from kitchen reactions to the light in the sky. These short facts explain matter, energy, electricity, magnets, sound, and the simple units we use to measure the world. The full list sits on the Science and Physics page, where every entry is listed in order.