How does a microwave heat food without an open flame?

A microwave oven uses electromagnetic radiation at a frequency of about 2.45 gigahertz to heat food. The magnetron tube inside the appliance generates these microwaves, which penetrate the food and cause water molecules to vibrate rapidly. Water molecules are polar, meaning they have positive and negative ends, so they constantly try to align themselves with the alternating electromagnetic field, which reverses billions of times per second. This rapid molecular friction generates heat throughout the food rather than just on the surface. This is fundamentally different from conventional cooking, where heat transfers from an external source inward. Because microwaves excite water molecules directly inside the food, heating happens faster and more evenly, though dense or thick foods can still have cooler centers.