How does popcorn pop? Are there any other foods that pop?

As the Industrial Revolution taught us, steam is powerful stuff. Inside the thick hull of a popcorn kernel is a jumble of proteins and starches called the endosperm, which also contains water. As you heat the kernels, that moisture turns to steam and the starch begins to “melt” into a goopy liquid, says Wendy Boersema Rappel of the Popcorn Board. The pressure builds and the steam and starch get hotter until—at around 347 degrees Fahrenheit and 135 pounds per square inch (that’s almost three times the pressure of your car’s tires)—the hulls burst and the liquefied starch explodes and rapidly cools and hardens, becoming what we recognize as popcorn.

There are actually two different popcorn shapes, each produced by different varieties of corn. Butterfly kernels are the familiar random pointy shapes you find at the movie theater and in microwaveable brands. Mushroom kernels are nearly round when popped and are more durable and better at holding seasonings. That’s why they’re often used in packaged prepopped corn, like cheese popcorn sold in vending machines.

Popcorn is only one of several types of corn. Not all corn can pop. The other three familiar types—sweet, the kind we eat on the cob; dent, the kind used for animal feed and in processed foods; and flint, the decorative kind you see at Halloween and Thanksgiving—don’t have thick enough or strong enough hulls to contain the pressures necessary to liquefy the starch before bursting, Rappel says.

There are a few other grains that can pop, however. Amaranth is a poppy-seed-size grain that you can find at health food stores and that made up an important part of the ancient Aztec diet. It pops just like popcorn and is mixed into a sweet syrup in a Mexican candy called alegría. It’s also possible to pop certain kinds of rice: essentially a homemade version of Rice Krispies cereal (although real Rice Krispies are puffed instead of popped, a similar process that involves heating rice or another grain under high pressure in what is essentially a gun barrel, then shooting it out to rapidly release the pressure).