First off, popcorn isn’t just any old corn. It’s a cultivated strain of flint corn known as zea mays everta. Its kernel is also a whole grain—it consists of the bran (the hull or outer covering), the germ (the “embryo” that germinates into a plant), and the endosperm (the starch that provides nutrition for the germ).
Inside the protective hull, each popcorn kernel is mostly some soft, starchy endosperm and a small amount of water. All three of those things help make popcorn pop the way it does. As you cook the kernels in a pan on the stovetop or in a microwaveable bag, the water inside each kernel starts to heat up, create steam and expand. The hulls keep all that energy contained, like little pressure cookers, for a short while. Popcorn hulls are great at this because their cellulose molecules are arranged in a highly-ordered pattern that makes them both strong and impermeable.
The water and steam eventually create enough internal pressure to break the hull and make the kernel pop, but there has to be a certain amount of water for it to pop right. Too little, and there won’t be enough energy to open the hull. The kernel will either be a dud, or steam will just escape from whatever cracks it could create. Too much water, and the final product will be dense and gummy instead of light and fluffy. Farmers harvest popcorn when its moisture content is between 16 and 19 percent by mass, and then dry it until the moisture drops into the 13 to 14 percent range.
As the pressure and heat are building up inside the kernel, they gelatinize and soften the starchy endosperm. When the kernel finally pops, the drop in pressure and the escaping steam cause the starch and proteins in the endosperm to expand, and the air outside the kernel rapidly cools them and causes them to solidify and set into white, puffy flakes that go great with butter and salt (or a little Old Bay seasoning).