From MinuteFood.

Depending on the direction and speed at which the foam expands, you can actually get four distinct popcorn shapes!

A kernel of popcorn is basically a bunch of starch and a little bit of water surrounded by a hard shell, which popcorn-ologists call the “pericarp” but you’ll generally hear referred to as the “hull”. And it’s the hull that makes popcorn special, because unlike other varieties of corn, which have pretty thin, weak hulls that make for tender eating, a popcorn kernel’s hull is way stronger and thicker. When you heat up that kernel, the starch inside starts to gelatinize – it basically turns into goo – and the water inside starts turning to steam, but the super-strong hull keeps holding everything together, allowing pressure to build up inside. Like, a lot of pressure; a popcorn kernel is basically an itty bitty pressure cooker. Except that most pressure cookers generally function at around 15psi, but the average popcorn kernel can withstand an internal pressure of more than 130psi before its thick hull finally fails. Once that happens, the starchy goo explodes into a foam, which almost immediately cools down and solidifies.
Depending on the direction and speed at which the foam expands, you can actually get four distinct popcorn shapes. If it expands more or less equally in all directions, you’ll get this – the “mushroom”. This shape is rare in home and theater popcorn; you’ll mostly see it made into flavored popcorn because it’s super sturdy. Instead, the starch in most commercial popcorn kernels expands more chaotically, creating a shape known in the popcorn biz as a “butterfly”. But butterflies can actually take on three distinct subshapes:. Even weirder is that these different shapes are markedly different in their texture, the amount of oil and salt they tend to pick up, and even their chemical composition, which means the proportion of the various shapes you get in a batch can really affect your eating experience.