How does a microwave oven heat food?

A microwave oven heats food by bombarding it with microwaves, a type of electromagnetic radiation produced by a device called a magnetron. These waves have a frequency, around 2.45 gigahertz, that is especially good at making water molecules rotate back and forth rapidly. This molecular motion is heat, so the food warms up. Because the waves penetrate a few centimetres into the food, microwaves heat from the inside outward to some extent, cooking faster than a conventional oven that heats only the surface. Foods with little water, and materials like glass or ceramic, absorb far fewer microwaves, which is why plates stay relatively cool while the food gets hot.