|Inside the microwave oven, what is it that heats the food? How does the heat come out; where did it come from?
The food is heated by the microwaves themselves and these microwaves are piped into the cooking chamber from the magnetron. The magnetron has electric charge sloshing back and forth in its tines. A small antenna uses that sloshing charge to emit microwave radiation. The water molecules in the food absorb this microwave radiation and turn its energy into heat. The usual rules of heat transfer don't apply in the heating process—the energy arrives at the food as microwaves, not heat.