First things first, you can't make just any type of corn pop.
When you heat the popcorn up, the tiny bit of water inside the kernel changes into steam and expands.
But the fun doesn't end there. There are actually two types of popcorn: mushroom and butterfly.
The two types form in slightly different ways.
With mushroom popcorn, the kernel expands evenly. But with butterfly popcorn, the starch inside comes out of wherever the kernel's skin breaks, basically turning it inside out. You can see butterfly popcorn forming in these GIFs: