Why popcorn pops, and why some kernels never do

A popcorn kernel is a tiny, sealed pressure vessel. Its hard shell traps water inside the starchy core until heat turns that water to steam, and pressure builds to roughly nine times normal air pressure before the shell finally gives way, at a strikingly consistent 180°C.

A 2015 high-speed-camera study caught what happens next: a short "leg" of molten starch kicks the kernel airborne, and the familiar pop sound arrives a few milliseconds later, from escaping vapour rather than the shell cracking.

Not every kernel makes it. A cracked or leaky shell lets steam slip away before the pressure ever builds, leaving a hard, unpopped "old maid" behind.