Lightning is caused by the build up of electrostatic charge in clouds. One region within the cloud builds up a positive charge and the other a negative charge. The process is not completely understood as to why, but the bottom of the cloud usually ends up being negatively charged and the top positively charged. If the build up (separation) of charge becomes great enough, the negative charges may leap to the positive side of another cloud, this is called sheet lightning or it may leap to the ground. The animation shows he creation of a ground strike of lightning.