Microwaves are super high frequency electro magnetic waves often used in communication systems.
Most of us meet microwaves when we cook in a microwave oven. Microwave ovens generate super high frequency waves and scatter them throughout the oven. The frequency of microwaves is just right to give energy to water molecules. Microwaves cause them to vibrate and collide with one another. This process changes the kinetic energy of the water molecules into heat that warms the food. Any thing containing water can be heated in a microwave oven.
You can boil water off objects like wet books or papers in your microwave oven. But they must never be used to dry living things because the vibration of water molecules inside the body would kill anything that was living.
Microwave ovens have a grating in the door which allow us to see our food cooking, while preventing us from being bombarded by the microwaves. The grating reflects the microwaves back into the oven. The microwaves, which have a wave length of about 12 centimeters, are too big to pass through the small holes in the grating. However visible light whose wave length is much smaller than the opening, can easily pass through the grating. The only way microwaves can leak out is through a badly fitting or dirty door seal.
So next time microwaves zap life into your leftovers, think of the action they are bringing into the lives of those little water molecules.