Popcorn or popping corn is a type of corn, which explodes from the kernel and puffs up when heated. Corn popping was originally discovered by Native Americans, but became popular as a snack food during the United States Great Depression, especially in movie theatres. Corn is able to pop because, unlike other grains, its kernels have a hard moisture-sealed hull and a dense starchy filling. 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 popping machines were invented by Charles Creators in the late 19th Century. Many types of small-scale home methods for popping corn also exist, with the most popular being prepackaged microwavable popcorn. As a snack food, popcorn has both advocates and detractors. Some consider it to be a health food, while others caution against it for a variety of reasons. Popcorn can also have non-food applications, ranging from holiday decorations to packaging materials.