Why does popcorn pop?

Popcorn pops because each kernel holds a little water inside a hard outer coat. Heat turns that water to steam, the steam gets trapped, and pressure rises until the coat breaks. When it opens, the hot starch inside expands into the white puff we eat.

The fun part is that popped corn is mostly airy starch foam that hardened in a blink. Kernels that stay small often had a crack or weak spot, so steam escaped early and the pressure never grew strong enough to burst them open.