By Joshua Engel, Enthusiastic eater and adventurous cook
There are actually two different shapes of popcorn, "butterfly" and "mushroom." In both cases, what happens when the popcorn pops is that the pressure causes a failure in the skin of the kernel. The kernel skin is actually quite tough; the hole expands, but the skin stays mostly together. The starches within are blown out through the hole, inverting the popcorn, so that the skin is now on the inside. It usually cracks into several pieces as it cools.
When the expansion happens evenly, you get mushroom popcorn. It's roughly spherical, though the outer surface will develop fissures. Some varieties of corn do not expand evenly. Corn kernels actually have several lobes, as seen here. And when these lobes expand away from each other, you get butterfly popcorn.
Several other factors figure into the exact shape of the popcorn kernel, including water and other growing conditions. Each crop will have some of each, but farmers can arrange to have mostly mushrooming or mostly butterfly popcorn. Surely, the time is ripe for foodies to declare a preference for one or the other and begin to seek out just the proper kind of popcorn.More questions on popcorn: