Lightning is a giant spark of static electricity, and thunder is just the sound that lightning makes. The process leads to lightning is the same one that produces a shock you sometimes get when you touch a doorknob or another person.

It's friction that's created between your feet and a carpet. That friction strips away electrically charged particles, called electrons, from the carpet and builds up an excess on the other object - you. Nature despises imbalance, and the little shock you get when you then touch something is nature's way of ridding your body of the excess electrical charge.

This process happens on a grand scale inside thunderstorms. The friction comes from collisions between hailstones and ice crystals inside the cloud. In those collisions, hailstones gain electrons and ice crystals lose them. The heavier hail tend to gather low in the cloud, the lighter ice crystals higher up. When the difference in electrical charge grows large enough - lightning is born within the cloud. And in fact, 80% of all bolts stay inside clouds - only about 20% ever strike the earth. We'll talk about that 20% tomorrow night.