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A microwave oven uses 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. Radio waves in this frequency range have an interesting property: they are absorbed by water, fats and sugars. When they are absorbed they are converted directly into atomic motion -- heat. Microwaves in this frequency range have another interesting property: they are not absorbed by most plastics, glass or ceramics. Metal reflects microwaves, which is why metal pans do not work well in a microwave oven.

Do not use metal pots and pans, or metal utensils. Food is completely cooked if the bottom of plate feels warm in the centre. Cook small foods, less than 2 inches in diameter, for less time, since the heat penetrates more quickly from all sides. To prevent food from drying out, cover the food with a vented cover whenever cooking or re-heating. Always use less salt and seasoning than normal when cooking in the microwave, since this draws out the moisture and will toughen the food. Add the salt later in the cooking process. Use less water when microwaving, due to the shorter cooking time. Stir liquids periodically when microwaving, and stir the outer sections, which cook faster, into the centre of the dish and centre sections to the outside. This equalizes the temperature throughout the food. Cook dense food for more time in the microwave, since dense food cooks slower than porous food. Arrange food with thickest parts, and bulky vegetables on outside of plate and quick-to-heat less-dense foods in the centre. Spread a single serving of a main dish in an even layer on plate. Turn large food over occasionally in the microwave, for more even cooking. Microwave cooked food require some standing time, since they continue to cook for a few minutes after they are removed from the microwave oven. Cover this food with plate, waxed paper or paper towel to direct the heat back into the food. Make sure not to block the vents of the microwave oven, or the oven will overheat.

The foods will be very hot when removed from the oven, so use oven pads and be careful. If the food is covered during cooking, make sure to leave a small portion vented, or uncovered, so steam doesn't build up and burn you when the covering is removed. The foods should sit as directed in the recipe after being removed from the oven so the heat can continue to spread and dissipate. This is called 'standing time', but it is actually more cooking time. Most ovens have hot spots, and if you eat the food directly from the oven, a few areas could be superheated and will burn. On the flip side, there can also be cold spots where the food doesn't get hot enough to kill bacteria. Follow stirring and rotating instructions carefully. Don't use metal containers unless the recipe specifically directs you to: as stated above, microwaves bounce off metal, which can cause arcing and a fire inside the oven. Some recipes may call for shielding parts of the food, especially meats, with small amounts of foil. This is perfect acceptable as long as the directions are carefully followed. Make sure any glass, plastic containers, and plastic wrap you use are labelled 'microwave safe'. Don't heat water or other liquids beyond the time recommended by the manufacturer or any recipe. Superheating can occur when plain water is heated in a clean cup for an excessive amount of time. The water will look innocuous, but when moved it can literally erupt out of the cup. Don't heat the water twice - that adds to the superheating risk. Adding sugar or coffee granules to the water will reduce the risk of superheating. Never operate a microwave if the door is damaged or doesn't close securely. Don't operate the oven while it is empty. This can also cause arcing and start a fire. It's also a good idea to stand 3-4 feet away from the microwave when it is operating - just to be on the safe side!

In microwave cooking, the radio waves penetrate the food and excite water and fat molecules pretty much evenly throughout the food. No heat has to migrate toward the interior by conduction. There is heat everywhere all at once because the molecules are all excited together. There are limits, of course. Radio waves penetrate unevenly in thick pieces of food (they don't make it all the way to the middle), and there are also "hot spots" caused by wave interference. The whole heating process is different because you are "exciting atoms" rather than "conducting heat."

Microwave ovens can definitely be harmful if used improperly. Microwave radiation can pass through plastic and glass, but it'll reflect off of metal. If you put a metal object (such as a metal bowl or fork) into the microwave oven, this can cause the microwaves to reflect back to the source that produces them, and can result in considerable damage to the oven.