According to the Popcorn Board (who knew there was a Popcorn Board?) October is National Popcorn Month.
Wikipedia explains popcorn thusly: Popcorn, also known as popping corn, is a type of corn (maize, Zea mays var. everta) that expands from the kernel and puffs up when heated. Corn is able to pop becauseits kernels have a hard moisture-sealed hull and a dense starchy interior. Pressure builds inside the kernel, and a small explosion (or “pop”) is the end result. Some strains of corn are now cultivated specifically as popping corns.
The Muppets explain it like this:
Gunsmoke and Knitting 