Lightning - Electrostatic charge
Lightning is caused by the build up of electrostatic charge in clouds. When all the electrons move to the bottom of the cloud, they leave the bottom half of the cloud negatively charged and the top half of the cloud positively charged. When the electrons move to the bottom of the cloud they become closer to the ground, pushing the electrons on the ground farther from the from the surface, leaving the protons closer to the cloud. The rule of electrostatic charge says that opposites attract, so, the electrons in the cloud shoot to the ground, to the protons, through something called lightning.