Why does popcorn pop?

 Each popcorn kernel contains a small amount of water stored in a circle of soft starch inside a hard outer casing. When heated to around 450 degrees F or 232 degrees centigrade, the moisture turns to steam, creating pressure within.

As the pressure builds, the casing eventually gives way, and the kernel explodes and pops, allowing the water to escape as steam and turning the kernel inside out.

This is similar to why, during seasoning, mustard seeds spurt in hot oil.