Each popcorn kernel seals water inside a tough, waterproof skin.
Heat turns the water to steam, building pressure until, at about 180°C (356°F), the skin bursts and the steam puffs the starchy inside into foam.
Kernels that never pop usually held too little water. Slow heating can fail too: steam seeps out of the tip, where the kernel was attached to the cob, before pressure builds.