What is the principle of making popcorn?

In the streets, sometimes you can see someone carrying a burden or pushing a car, specializing in selling popcorn. It is generally believed that when the iron cylinder filled with popcorn is heated, the rice grains inside are slowly expanding and getting bigger, but this is actually a wrong understanding. When the iron drum is heated, the rice grains in the iron drum are not much bigger than when they are put in. Just at the moment when the lid was suddenly opened, the rice grains in the iron drum suddenly became "fat" because of the pressure in the iron drum.