Why does popcorn pop?

Each popcorn kernel contains a small amount of water, roughly 14 percent by weight, trapped inside a dense starchy center and enclosed by an unusually hard, moisture-sealed hull called the pericarp. Heating turns that water to steam, and because the hull does not let it escape, pressure builds to around 135 pounds per square inch and the interior starch gelatinizes into a pressurized gel at temperatures near 180 degrees Celsius. When the hull finally fails, the sudden pressure drop causes the gel to expand explosively to many times its original volume and cool almost instantly, setting into the familiar foam structure. High-speed footage shows the kernel launching itself off the surface as it turns inside out.

This is why only one variety works. Popcorn is a specific type of flint corn whose hull is strong and impermeable enough to hold pressure to the necessary point, while sweet corn, dent corn, and field corn have hulls that leak or rupture too early and merely scorch. Moisture content is the critical variable in practice: kernels that have dried out below roughly 12 percent cannot generate enough steam and remain unpopped, which is why old popcorn produces more duds and why storing it in an airtight container rather than an open bag noticeably improves results. Adding a small amount of water to stale kernels and resting them for a few days can partly restore performance.

Practical implications for a better bowl. Heat evenly and preheat the oil, since slow heating lets moisture escape gradually and produces more unpopped kernels. Stop when popping slows to a few seconds between pops rather than waiting for silence, because the burning point arrives quickly. Microwave bags rely on the same physics with a susceptor that concentrates heat. Two health notes worth knowing: air-popped popcorn is a whole grain and a reasonable snack before anything is added to it, and the artificial butter flavoring compound diacetyl, associated with severe lung disease in factory workers, was removed from most commercial microwave popcorn following that finding.