One of the first things ever cooked in a microwave was popcorn. To be historically accurate, popcorn was actually the second thing. The very first thing ever heated up by microwave energy was an employee at the Raytheon Corporation and a candy bar in his pocket in 1945.

If you examine a popcorn kernel closely, you will notice that it is completely sealed with a hard outer layer. Inside this protective outer shell is the corn itself and moisture. When popcorn kernels are placed in a microwave oven, the microwave energy starts the molecules in the moisture located inside the popcorn kernel to vibrate rapidly causing friction. The friction is so intense that the moisture inside the kernel changes from a liquid to a gas. In other words, it gets turned into steam. This steam creates such intense pressure that the outer kernel of the popcorn explodes. That is the popping sound you hear when popcorn is popping. On the stove, in a hot air proper, or in oil, the bottom line is that steam is what makes popcorn pop.

Why are there always leftover kernels uncooked when cooking popcorn?

The reason that there is always popcorn kernels left over is because some of the kernels will have cracks in the outer shell. When this happens the moisture in the shell dries up, leaving nothing behind to turn to steam. Leaving the microwave oven on longer, in an effort to try to pop these left over kernels, simply won't work. In fact, doing so will diminish the quality of the popcorn that has already popped.

How does the popcorn sensor work on a microwave oven?

The popcorn sensor on a microwave oven is designed to sense the steam that is coming out of the kernels. When there is no longer any steam left in the microwave oven cavity, the sensor turns off the microwave oven and beeps to let you know it's time to eat popcorn. Using the popcorn sensor on a microwave will actually increase the life of the microwave oven. That's because the sensor will only let the microwave operate the oven long enough to cook all the popcorn kernels that still have moisture in them. When a microwave oven operates with little or no load in the oven cavity, that microwave energy is reflected back toward the magnetron. When this happens, the magnetron will deteriorate and fail prematurely.