Question: How is popcorn categorized by variety?
About the article "Popcorn", section "Expansion and yield".

Candidate passages:
Popcorn seasoning can refer to a variety of seasonings that are used to add flavor to popcorn.
In the United States, popcorn seasoning is mass-produced by several companies for commercial and consumer use.
Popcorn seasonings may be used to enhance the flavor of popcorn, and some are used to add a buttery flavor to popcorn.
Significant amounts are often used to ensure the adequate flavoring of popcorn, due to popcorn's low density.
It is also sometimes utilized to add coloring to popcorn.
Some popcorn seasoning may contain monosodium glutamate.
Some specialty products exist in unique flavors, such as chocolate and bubble gum.
Dry popcorn seasoning may be finely granulated to enable even dispersion upon popcorn.
Some oils used to cook popcorn contain popcorn seasonings mixed within the oil, and may be referred to as popcorn seasoning oils or liquid popcorn seasoning.
Common homemade popcorn seasoning ingredients include salt and melted butter.
Some popcorn seasoning products may be referred to as popcorn salt. 
Popping results are sensitive to the rate at which the kernels are heated.
If heated too quickly, the steam in the outer layers of the kernel can reach high pressures and rupture the hull before the starch in the center of the kernel can fully gelatinize, leading to partially popped kernels with hard centers.
Heating too slowly leads to entirely unpopped kernels: the tip of the kernel, where it attached to the cob, is not entirely moisture-proof, and when heated slowly, the steam can leak out of the tip fast enough to keep the pressure from rising sufficiently to break the hull and cause the pop.
Producers and sellers of popcorn consider two major factors in evaluating the quality of popcorn: what percentage of the kernels will pop, and how much each popped kernel expands.
Expansion is an important factor to both the consumer and vendor.
For the consumer, larger pieces of popcorn tend to be more tender and are associated with higher quality.
For the grower, distributor, and vendor, expansion is closely correlated with profit: vendors such as theaters buy popcorn by weight and sell it by volume.
For both these reasons, higher-expansion popcorn fetches a higher profit per unit weight.
Popcorn will pop when freshly harvested, but not well: its high moisture content leads to poor expansion and chewy pieces of popcorn.
Kernels with a high moisture content are also susceptible to mold when stored.
For these reasons, popcorn growers and distributors dry the kernels until they reach the moisture level at which they expand the most.
This differs by variety and conditions, but is generally in the range of 14–15% moisture by weight.
If the kernels are over-dried, the expansion rate will suffer and the percentage of kernels that pop at all will decline.
When the popcorn has finished popping, sometimes unpopped kernels remain.
Known in the popcorn industry as "old maids", the kernels don't pop because they do not have enough moisture to create enough steam for an explosion.
Rehydrating prior to popping usually results in eliminating the unpopped kernels.
Popcorn varieties are broadly categorized by the shape of the kernels, the color of the kernels, or the shape of the popped corn.
While the kernels may come in a variety of colors, the popped corn is always off-yellow or white as it is only the hull (or pericarp) that is colored.
"Rice" type popcorns have a long kernel pointed at both ends; "pearl" type kernels are rounded at the top.
Commercial popcorn production has moved mostly to pearl types.
Historically, pearl popcorns were usually yellow and rice popcorns usually white.
Today both shapes are available in both colors, as well as others including black, red, and variegated.
Commercial production is dominated by white and yellow.
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.
Popcorn is able to pop because, like amaranth grain, sorghum, quinoa, and millet, its 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.
There are various techniques for popping corn.
Along with prepackaged popcorn, which is generally intended to be prepared in a microwave oven, there are small home appliances for popping corn.
These methods require the use of minimally processed popping corn.
A larger-scale, commercial popcorn machine was invented by Charles Cretors in the late 19th century.
Unpopped popcorn is considered nonperishable and will last indefinitely if stored in ideal conditions.
Depending on how it is prepared and cooked, some consider popcorn 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. 
The Weaver Popcorn Company, based in Van Buren, Indiana, is one of the largest popcorn companies in the United States.
Founded in 1928 by Ira E. Weaver, whose family still controls the company, it develops, grows, processes, packages, and ships a variety of popcorn products for sale around the world.
Its customers include store chains around the world, as well as concessionaires, and international popcorn distributors.
In 2007, the Weaver Popcorn Company became the first company to remove diacetyl, a controversial butter flavoring, from its "Pop Weaver" microwave popcorn products. 
Popcorn is located at .