They way it works is this: each kernel of popcorn contains its own tiny troll. When you get the kernel hot the troll gets so angry it explodes out of its little corn house and tries to take revenge by scalding you with a searing hot puff of steam.
An alternate explanation is that it’s the popcorn kernel’s thick outer membrane — its pericarp — that is the key to its popability. The popping of a kernel is a literal explosion. Heat penetrating the kernel from the outside causes a steam buildup on the inside. That steam creates pressure, pressure that increases the more the kernel heats. Finally, when the temperature reaches about 350 degrees and the outward pressure on the pericarp is in the arena of 135 pounds per square inch, the pericarp gives way.
When that happens, the hull’s contents — now a hot gel composed mostly of starch with a few protein molecules mixed in — burst outward. Expanding steam pushes the compact gel out into a diffuse cloud, which cools on contact with the outside air, freezing it into a foam. Neat.
The trick, as with any type of bomb, is to keep the explosive contents tightly confined for as long as it takes to build up the necessary pressure. A firecracker with a hole in it fizzles. In the very same way a pericarp with a hole or a weak spot cracks or gives way too soon, relieving the inside pressure and ruining the pop. Popcorn is the only type of corn that has this sort of thick, pressure-resistant pericarp, though even among popcorn kernels there are always some duds, as anyone who’s ever cracked a filling in the movie theater knows. This is why popcorn companies are forever looking for varieties of popcorn with better pericarps, with crystal cellulose structures that are more consistent and even, with fewer weak spots.
That or angrier trolls, however you want to look at it. Because honestly, the science isn’t really settled on this.