Thunder sounds as if huge things were crashing together in the sky. That is just what happens. Huge quantities of air do crash together during a thunderstorm. Lightning is the cause of the crashing.
As lightning jumps across the sky, it heats the air. The air grows hot so suddenly that it gives a terrific push outward. This leaves an almost empty space along the lightning's path. A moment later cold air rushes in to fill the space. Air from one side bumps into air from every other side. There is so much bumping that it makes a tremendous noise.
Thunder rolls and surges and rumbles. That is because the lightning jumps in uneven flashes. Sometimes thunder has extra rumbles. These are echoes that come from hills and valleys and from the clouds themselves.
