Believe it or not, a microwave oven uses friction to cook everything. The magnetron in the microwave along with the high voltage transformer, the diode, and the capacitor create an electromagnetic wave that causes the molecules in the food inside a microwave oven cavity to vibrate extremely fast. The energy from the magnetron actually causes the molecules to switch polarity so fast that they vibrate, cause friction, and heat up. Let's look at it in a simplistic way.
Everyone knows that a compass points to the magnetic north of the earth in the United States. It is very easy to influence the needle on a compass by putting a magnet close to it. As soon as the magnet gets close, the needle will swing toward the magnet. Now imagine for a second that you had two magnets and you quickly put one magnet at the top of the compass, pulled it away and put another magnet at the bottom of the compass. You would see the needle swing to the top and very quickly back down to the bottom. Now imagine for a second that you could do this 24 million times in one second. Obviously that is not possible, but you would have to agree that the bearing inside the compass that the needle was attached to would get very hot rather quickly because of the friction of the magnetic fields changing it's direction so quickly. And believe it or not, that is exactly how a microwave oven cooks.
Now let's look at it another way. Take the palm of your hand and put it on your pant leg and move it back and forth very quickly. Make sure you are not sitting at your desk when you do this because you will undoubtably get some odd looks from your coworkers. If you move the palm of your hand back and forth on the material of your pants, you will notice that your palm actually starts to get warm. If you do it fast enough and long enough, you can actually burn your hand. That is friction, and that is exactly how microwave ovens cook.
How Does Popcorn Cook in a Microwave Oven
We are back to the concept of friction generating heat again. A popcorn kernel is sealed very tight. And inside in addition to the material that we eat, there is moisture. When a microwave oven generates energy it causes the moisture molecules also known as water, inside the popcorn kernel to vibrate violently causing it to heat up and change from a liquid to a gas. The gas is the common form of steam. The steam put so much pressure on the inside of the popcorn kernel it erupts in an effort to escape. That eruption as the steam escapes makes the sound we hear when popcorn pops.