Popcorn or popping corn is a type of maize which puffs up when it is heated in oil or by dry heat. Special varieties of corn are grown to give improved popping yield. Some wild types will pop, but the cultivated strain is Zea mays L. subsp. mays (Everta Group), which is a special kind of flint corn. It is a popular snack food all over the world.

In the United States, popcorn is particularly common in movie theaters. It is often served with butter and salt. It is also sometimes flavored with sugar (especially in Europe) or spices. In Sweden it is sometimes served with sugar and vinegar.

Popcorn balls (popped kernels stuck together with a sugary "glue") are a traditional Halloween treat. Cracker Jack is a popular, commercially produced candy that consists of peanuts mixed in with caramel-covered popcorn.

Four popular brands of popcorn in the United States are Orville Redenbacher, Act II (both of which are brands of the ConAgra Foods conglomerate), Pop Secret and Black Jewell, a premium specialty brand.

How popcorn pops

As with all grains, each kernel of popcorn contains a certain amount of moisture in its starchy endosperm. Unlike most other grains, the outer hull or pericarp of the popcorn kernel is thick and impervious to moisture.

As the kernel is heated past the boiling point, this water begins to turn to steam. In kernels of other grains (and in damaged kernels of popcorn), this steam escapes as fast as it appears, but in the tightly sealed popcorn kernel, the steam is held tight by the pericarp and the pressure starts to build.

Eventually, the pressure is higher than the pericarp can handle, and there is a small explosion. The force of the explosion turns the kernel inside out. More importantly, because the moisture is evenly distributed throughout the starchy endosperm, the sudden expansion turns the endosperm into an airy foam, giving popcorn its special texture. Unpopped kernels are referred to in the industry as "old maids", and are a subject of ongoing research in getting as many kernels to pop as possible.

The key to this phenomenon is the sudden transition from a high pressure to a low pressure. Other grains can be popped by artificially inducing this transition, heating the kernels in tightly sealed container, and then suddenly releasing the seal.

Popcorn facts

• Popcorn has been served in movie theaters since 1912.
• Buttered popcorn sells twice as well as plain popcorn.
• The first popcorn, made by Native Americans, was flavored with dried herbs & spices.