Once you pop in your food and hit start, Marshall Brain explains that a microwave oven has a little metal box inside of it called a magnetron. The magnetron produces the microwaves that heat your food. And microwaves, says Marshall, are really radio waves. A little metal fan inside your microwave breaks up the radio waves and spreads them throughout the oven. Some ricochet off the walls, some hit the food directly.
Marshall points out that radio waves are in the air all around us. But in the case of microwave ovens, the radio waves have a specific frequency that gives them an interesting property: they are absorbed by water, fats and sugars. Food contains those same molecules that become excited as the radio waves strike them. This excitement turns into heat, which cooks your food.