How does popcorn burst and flip inside out?

Popping corn is about 13.5 to 14 percent water by weight. As a kernel heats in a pot or microwave, that water tries to turn into steam, but the hard, tight hull keeps the steam from escaping, so the pressure inside keeps rising. In 2015, French researchers Virot and Ponomarenko confirmed that almost all kernels pop at about 180°C. When the hull finally gives way, the trapped steam puffs up the starch in an instant and turns the kernel into an inside-out white mass. The puffed starch cools right away and sets crisp.

Many people think the pop is the sound of the hull cracking. But when Virot and Ponomarenko matched a camera shooting 2,900 frames per second with sound recordings, the sound started a few thousandths of a second after the hull split. So the researchers explained that the noise comes from trapped steam rushing out of the crack all at once. Think of the pop you hear when you open a bottle.

Popcorn hopping around the pot is not caused by steam shooting out like a rocket either. In the same videos, part of the starch of a popped kernel stuck out like a leg and pressed against the bottom, then sprang back like a spring and launched the kernel. The kernel did not move at the moment the steam escaped. The researchers compared popping to plants that burst open their seed pods, and jumping to animals that leap on their legs.

Unpopped kernels left at the bottom of the pot are usually short on water or have a hull that leaks steam. The Popcorn Board says kernels dry out easily when left uncovered, so it recommends keeping them in an airtight container in a cool place. Dried-out kernels can often pop well again after a few days in a jar with a little water, shaken together.