Why popcorn pops

Popcorn pops because moisture inside a dense starchy kernel becomes steam, and a hard hull holds the pressure until it fails all at once. The softened starch expands with that steam and then sets as the foam.

The kernel is not a pocket of air that swells, and it does not pop because the shell burns open. The working fluid is water trapped in the endosperm, so a kernel that has lost that moisture, or whose hull is already cracked, never builds the pressure.

The mechanism is more particular than the slogan. Heat softens the starch while the intact pericarp keeps the internal water from leaving, so the water becomes steam under rising pressure. When the hull ruptures, the pressure drops suddenly and the steam blows the hot starch into a foam of tiny bubbles.

Follow it one step further. The foam then cools and the starch rigidifies, so it does not collapse back into a dense seed. The sound is the rupture, and the white shape is the starch the steam inflated and left behind.

The case worth keeping in mind is specific. A pot of kernels is quiet and then abrupt, each pop one hull failing rather than a slow inflate. The leftovers that never pop are often kernels that leaked steam early or did not hold enough moisture to reach bursting pressure.

That case has a boundary. Ordinary field corn and most other grains lack a pericarp that can hold this pressure around a starch ready to foam, so they scorch or split weakly instead of turning inside out. Popcorn is a particular kind of maize, and heat by itself does not recruit the others.