Popcorn, like all grains, contains water.
About 13.5 – 14% of each kernel is made up of water. So when a popcorn kernel is heated above the boiling point of 212 degrees Fahrenheit, this water turns to steam. This steam creates pressure within the kernel, causing the kernel to explode and turn itself inside out.
But, if the water inside a piece of popcorn is what makes it pop, why don't other grains pop as well? Wheat and rice contain water, so why don't we sit down to watch a movie with a bucket of popped rice or popped wheat?
The answer lies in the differences between the outer coverings, called hulls, of popcorn and other grains. Unlike rice and wheat, and unlike even regular corn, popcorn has a non-porous hull that traps steam. With the porous hulls of other grains, steam easily passes through, so no significant pressure is produced. These grains may parch, but they will not pop.
But even popcorn, with its special hull, doesn't always pop. Popcorn must have two important properties to pop well. First, the amount of moisture in the kernel must be very close to 13.5%. Too little moisture and enough steam won't build up to pop the kernel. Too much moisture and the kernels pop into dense spheres, rather than the light fluffy stuff popcorn fanciers love.
Second, the kernels must not be cracked or damaged in any way. Even a small crack will let steam escape, keeping the necessary pressure from building. Popcorn kernels with the right amount of moisture and unblemished hulls pop into the snack that just about everyone enjoys.