Each popcorn kernel contains a small amount of moisture trapped inside its hard outer shell. When the kernel is heated, this moisture turns into steam, creating pressure within the kernel. As the pressure builds up, the hull eventually bursts open (POP! ), releasing the steam and causing the starchy inside to rapidly expand and puff up, forming the familiar popcorn shape.

Some fun facts

1. Popped popcorn typically fills about 40 times the space it did as a pile of kernels.
2. Popcorn kernels can pop up to 3ft in the air.
3. The "world's largest" popcorn ball (made of corn syrup, sugar and popcorn) is 12ft in diameter and weighs 9370 lbs. You can visit it in Sac City Iowa.
4. Popcorn is a whole grain. In addition to fiber, popcorn is an amazing source of antioxidants (polyphenols) that have been linked to better circulation, digestive health and potentially lowering the risk of certain cancers.