This man may be smiling as he puts the aluminum foil in the microwave, but he won't be smiling for long.

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The microwave oven is one of the great inventions of the 20th century -- you can find them in millions of homes and offices around the world. At one time or another, we've all been told not to use metal products, especially aluminum foil, when cooking­ with a microwave oven. Stories of incredible explosions and fires usually surround these ominous warnings. Why is that? Let's take a look at how microwave ovens work to find out.

As incredible as microwave ovens are, the technology behind them is fairly simple. Microwave ovens use microwaves to heat food. Microwaves are radio waves. In the case of microwave ovens, the commonly used radio wave frequency is roughly 2,500 megahertz (2.5 gigahertz). Radio waves in this frequency range have an interesting property: Water, fats and sugars absorb them. When absorbed they're converted directly into atomic motion -- heat. Microwaves in this frequency range have another interesting property: Most plastics, glass or ceramics don't absorb them. But what about metal?

The walls inside a microwave oven are actually made of metal. It turns out that a fairly thick piece of metal works a lot like a mirror. But instead of reflecting an image, it reflects microwaves. If you were to put food in a heavy metal pan and put it in the microwave, it wouldn't cook. The pan would shield the food from the microwaves, so the food would never heat up.

Other metals are a no-no in the microwave, too.

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Tiny sharp pieces and thin pieces of metal are a different story. The electric fields in microwaves cause currents of electricity to flow through metal. Substantial pieces of metal, like the walls of a microwave oven, can usually tolerate these currents without any problems. However, thin pieces of metal, like aluminum foil, are overwhelmed by these currents and heat up very quickly. So quickly in fact, that they can cause a fire. Plus, if the foil is crinkled so that it forms any sharp edges, the electrical current running through the foil will cause sparks. If these sparks hit something else in the oven, perhaps a piece of wax paper, you'll probably be reaching for the fire extinguisher.

While it's highly unlikely that a small piece of foil is going to cause your microwave oven to totally explode, it could cause a fire. So, it's a good idea to stick to plastic wrap, paper towels and any other non-metal kitchen aids.