Why shouldn’t metal objects be placed inside microwave ovens?
Since water molecules are warmed and eventually boil off by microwaves, anything that is wet can be dried in the microwave. However, there is one very important consideration that must be made before placing the object inside a microwave—the object being dried must not contain a great deal of water itself. Microwaves are wonderful at drying wet books, papers, and magazines, but must never be used to dry things like plants or small animals. Living things would be killed by the resonance of water molecules inside their bodies.
Manufacturers caution consumers about placing metal containers and aluminum foil inside microwave ovens for two main reasons. The first reason is that metal and aluminum may impede cooking. Microwaves warm food by transmitting energy to water and fat molecules within the food. If food is placed under aluminum foil or in a metal container, the microwaves will be reflected from the metal and won’t be able to reach the water molecules and cook the food.
The second reason for not placing metal objects inside a microwave oven is for the safety of the microwave oven itself. Metal acts as a mirror to microwaves. If too much metal is placed in the oven, the microwaves will bounce around the oven in waves that can damage the magnetron that produces the microwaves. If the metal is the correct size it can act as a microwave receiver and the induced voltages can produce sparks that can ignite the food.