Lightening occurs when electricity bulids up in thunder clouds, and then is released as lightening. It is very hot, and is shaped like a zig-zag. Lightnning does not always go from the cloud down to Earth, it can also run from cloud to cloud. Lightening takes the shorest path between the sky and the ground, which often causes it to strike trees. Thunder makes a huge booming sound.
Thunder is caused by the expension of air at high speeds.
As the air expends, it causes the booming noise. Thunder can be heard from as far as ten miles away. Thunderstorms can be scary because of the loud boom.