There is a small amount of water found in every kernel of popcorn, and during the cooking process, this water initially turns to steam, which leads to a severe increase in pressure that causes the kernel to pop. The amount of water in every kernel is between 13.5 and 14 percent.
There are four main types of corn that are typically grown: sweet corn, which is what humans eat from the cob or in fresh, canned or frozen niblet form; field or dent corn, which serves as feed for many farm animals; Indian or flint corn, which is often dried and serves as decoration; and popcorn. Of the four types, popcorn is the only one that has a thin enough hull that allows the kernels to burst open.
During the cooking process, the water inside the popcorn kernels, which is encased in soft starch inside each one, turns to steam at 212 degrees Fahrenheit. At this point, the composition of the starch changes into a softer, gel-like consistency. The heating continues until it reaches 347 F. At this point, the kernels pop, releasing the steam as it bursts through the thin hull, inflating the starch as it happens. Then, because the exterior is much cooler than it was inside the kernel, the starch cools immediately into unique-looking shapes.