A Snack That Explodes
Popcorn is one of the few foods that puts on a small show while it is being made. Hard little kernels go into the heat, and moments later they burst with a pop into soft, fluffy white pieces many times their original size. It can seem almost like magic, but the popping of popcorn is a neat demonstration of basic physics. The secret is hidden inside each tiny kernel, in the form of a small amount of water.
Water Locked Inside the Kernel
Every popcorn kernel contains three important parts: a hard outer shell called the hull, a starchy interior, and a small amount of water trapped within that starchy center. The amount of water is small, only a little of the kernel's weight, but it is essential. Not all corn can pop, and one big reason popcorn is special is that its hull is unusually hard and seals in moisture well. This trapped water is the fuel for the entire popping process.
Building Up the Pressure
When a popcorn kernel is heated, the water trapped inside begins to heat up too. As the temperature climbs high enough, the water turns into steam. Steam takes up far more space than liquid water, so it tries to expand. But the hard, sealed hull does not let the steam escape easily. As a result, pressure builds up inside the kernel, growing stronger and stronger as the heating continues. The kernel becomes like a tiny, sealed pressure container.
The Big Pop
Eventually the pressure inside becomes too great for the hull to contain. At a high enough temperature, the hull suddenly bursts open. The trapped steam rushes out, and the soft starchy interior, which has become soft and gelatinous from the heat, rapidly expands into a foam. As it cools, this expanded starch sets into the light, fluffy shape of popped popcorn, essentially turning the kernel inside out. The familiar popping sound comes mostly from the burst of steam escaping the kernel.
Source
This article was written using information from Wikipedia.