Microwave ovens are wonderful, revolutionary cooking devices. But there are some things that you cannot, or should not, do with microwaves. For instance, you should never put animals inside, and never stare up close directly into a working oven for longer than a few moments at a time. Finally, never put metal such as aluminum foil into a microwave oven. Besides preventing the microwaves from reaching and heating up the food it conceals, aluminum foil tends to give off sparks that might start a fire.
As its name implies, a microwave oven uses intense microwaves to heat and cook food. Like any electromagnetic wave, a microwave consists of electric and magnetic fields. When the strong microwaves produced by a microwave oven interact with a sheet of aluminum foil, their electric fields cause the mobile electrons in the foil to accumulate at the sheet’s sharp, pointed tips. This build-up of electrons creates a strong electric field in the surrounding air.
When the electric field becomes strong enough, it causes free electrons in the air to accelerate and jar loose more electrons from gas molecules in the air, which then go on to jar even more electrons loose. This chain reaction creates an electrical charge in the air, which shows itself as sparks. Although rounded metal objects such as spoons are less likely to create sparks, the safest bet is to avoid putting any kind of metal in the microwave.