A tube within the oven, called a "magnetron", emits high-frequency electromagnetic waves (similar to radio waves). This radiation is scattered in the oven by a fanlike reflector (called the "stirrer"). When the waves penetrate the food, they reverse the polarity of the water and other liquid molecules, billions of times a second. This oscillation causes the molecules to vibrate and bounce against each other. These collisions create friction and, as a by-product, the heat that cooks or warms the food.
The microwave's heating element does not heat the circulating air, the oven walls, or the vessel holding the food. When a bowl or plate becomes warm in a microwave oven, either it absorbed the heat from the cooking food or the surface of the vessel not touching the food was wet before the oven was turned on.