Sometimes as the sun warms the Earth, clouds form. In some conditions the clouds can form tall clouds called cumulus clouds. These clouds can actually rain or hail from inside the top of the could down to the bottom of the cloud. The movement of rain or hail inside the cloud makes the top of the cloud have a positive electrical charge and the bottom of the cloud have a negative electrical charge.
If the difference between the positive charge and negative charge becomes too great, it causes an electrical spark or lightning. Most lightning doesn't actually hit the ground. It just goes from the upper part of the cloud to the lower part. Sometimes it touches the earth.
Lightning is very hot. And it is formed very fast. Because the heat from lightning is created very fast, it heats the air around it very fast. The rapid change in air temperature creates a wave. Just like a wave through water, the hot air wave moves across the sky, but faster.
As the air cools back down, behind the wave of hot air, it makes the sound known as thunder. Thunder is created by a vacuum -- it is the very rapid equalization of air pressure. Essentially, the superheated air cools very quickly and as it cools the air collides with itself. It is this collision of rapidly cooling air that causes thunder.
