There are six main types of corn, but only popcorn will pop when it is heated. What makes popcorn so special? Popcorn kernels have an especially hard exterior (called a hull) that seals in moisture. The inside part of a popcorn kernel contains a small droplet of water surrounded by a dense, starchy material. When the kernel is heated, the water droplet turns into steam. As the steam builds, pressure increases within the kernel. Soon, the pressure becomes so great that the kernel explodes from the inside out, transforming into a fluffy, crunchy movie snack.