Once inside the oven, a microwave reflects off the metal wall until it strikes the food. Food cooks when microwaves penetrate the surface and start vibrating the water molecules in the food. The higher the water content, the more evenly the food will heat. This constant motion causes friction, which heats the food. All microwave ovens are electric, and most sit on a counter top.
Some units have a shelf in the cavity, which allows more food to cook at one time.
But, as cooking quantity doubles, cooking time also nearly doubles. A dial or touch-pad controls the time and power. All microwave ovens have interlock switches that turn off the oven if the door is opened. A standard size microwave oven is perfect for slow periods and for cooking individual or small portions. As the quantities get larger, a microwave oven saves little time or energy.