How does a microwave oven work?
(Lansing State Journal, November 15, 1995)
Now, let's step away form the water molecule for a moment, and consider a car. At a certain velocity, some cars tend to vibrate. This shaking is felt only at a specific speed. Something in the car becomes unbalanced as the engine reaches a certain frequency (races faster, or RPM). Water molecules, like metal in a car, have specific resonant frequencies where increased vibrations can occur.
A microwave oven is like an antenna at a radio station. It emits electromagnetic waves at one of the frequencies that water molecules are tuned to. As the water molecules begin to vibrate and rotate, they bump into other molecules in, say, a slice of pizza. The molecules speed up, the heat increases, and your pizza is ready to eat!