There are many things about lightning that are not understood. Lightning has been seen in the atmospheres of Venus, Earth, Jupiter, and Saturn. A leading theory is that collisions of particles in clouds cause large areas of positive and negative charge. When large oppositely charged areas get close enough together, electrons and/or ions race between them and create a path where more charged particles can follow - lightning. On average, over 100 lightning strikes occur every second over the surface of the Earth. Here lightning strikes near a Space Shuttle before launch. Lightning can be extremely dangerous - stay out of open areas during thunderstorms.