Popcorn, or popping corn, is corn (maize) which expands from the kernel and puffs up when heated. Corn is able to pop because, like sorghum, quinoa and millet, its kernels have a hard moisture-sealed hull and a dense starchy interior. This allows pressure to build inside the kernel until an explosive "pop" results. Some strains of corn are now cultivated specifically as popping corns. There are many techniques for popping corn. Commercial large-scale popcorn machines were invented by Charles Cretors in the late 19th century. Many types of small-scale home methods for popping corn also exist, with the most popular in the USA being prepackaged.
Popcorn was first discovered thousands of years ago by Native Americans in North America. It is one of the oldest forms of corn: evidence of popcorn from 3600 B.C. was found in New Mexico. The English who came to America in the 16th and 17th centuries learned about popcorn from the Native Americans.
During the Great Depression, popcorn was comparatively cheap at 5–10 cents a bag and became popular. Thus, while other businesses failed, the popcorn business thrived and became a source of income for some struggling farmers. During World War II, sugar rations diminished candy production, causing Americans to eat three times more popcorn than they had before.
The world's largest popcorn ball was unveiled in October 2006 in Lake Forest, Illinois. It weighed 3,415 pounds (1550 kilograms), measured 8 feet (2.4 m) in diameter, and had a circumference of 24.6 feet (8 m).