Why does corn turn into popcorn?

Corn kernels are wrapped in film. After being heated at high temperature, the water inside the corn began to evaporate and its volume expanded rapidly, but the external film tried to prevent it from expanding. As a result, the pressure inside the corn is getting bigger and bigger. Finally, the film could not be wrapped and burst, but ordinary rice needs to be burst in a special pressure cooker (because there is no such film outside the rice).