Popping corn is special breed of corn that is designed to pop. Popcorn is what you get when you heat popping corn and the kernels explode and turn inside-out. Popping corn pops because the corn kernels have a hard shell and are completely sealed. When they are heated, the water inside the kernel turns to steam but the steam can't get out. As you continue to heat them, the pressure of the steam continues to build up until the shell can't hold it anymore and the kernel explodes with a loud pop. As the steam is all through the starch inside of the kernel, the explosion makes the popped corn much larger than the unpopped kernel, and as it cools down so fast after the explosion the texture is that of a crispy puff.