Corn (maize) is nearly unique among grass species in that its seed has a strong, moisture-resistant hull, surrounding an interior composed almost entirely of hard, dense starch. It is this combination of traits that causes popcorn to pop.
Popcorn is a specialized breed of corn known as flint corn, which has an especially hard outer kernel. When popcorn kernels are heated in a pot, the steam cannot escape through the moisture-proof kernel, it soon becomes pressurized and then superheated.
This superheated steam cooks the hard starches in the kernel into a soft, gelatinous mass. A constant influx of heat from the pot keeps increasing the heat and pressure inside the kernel up to roughly 135 pounds per square inch, at which point the kernel finally ruptures.
The released steam then expands rapidly, filling the gelatinized starch with air. With the steam gone, the starch cools rapidly and solidifies into the familiar popped corn shape.
Popcorn noisily testifies to "praise" it's divine Creator Who came up with such an idea, another unique food to delight us–His beloved human family.
Adapted from:
http://en.wikipedia.org/wiki/Popcorn http://en.wikipedia.org/wiki/Flint_corn