1946, Spencer accidentally discovered microwave melting candy. Facts have proved that microwave radiation can cause molecular vibration inside food, thus generating heat. 1947, the first microwave oven came out. But it is only in recent years that people use microwave ovens to cook and cook. Microwave is an electromagnetic wave. The energy of this electromagnetic wave is not only much larger than that of ordinary radio waves, but also has a "personality": microwave will be reflected when it comes into contact with metal, and metal can't absorb it or conduct it at all; Microwave can pass through insulating materials such as glass, ceramics and plastics, but it will not consume energy. For food containing water, microwave cannot penetrate, but its energy will be absorbed. Microwave ovens are made by using these characteristics of microwaves. The shell of the microwave oven is made of metal materials such as stainless steel, which can prevent the microwave from escaping from the oven, so as not to affect people's health. Food containers are made of insulating materials. The heart of microwave oven is magnetron. This electron tube, called magnetron, is a microwave generator, which can generate microwaves with a vibration frequency of 2.45 billion times per second. This invisible microwave can penetrate the food to a depth of 5 cm and make the water molecules in the food move with it. Intense exercise produces a lot of heat energy, so the food is cooked. This is the principle of microwave heating. When cooking food with an ordinary stove, heat always gradually enters the food from the outside. When cooking in a microwave oven, the heat directly enters the food, so the cooking speed is 4 to 10 times faster than other stoves, and the thermal efficiency is as high as 80%. At present, the thermal efficiency of other stoves can not be compared with it!