Lisa emailed a question that seems questions a stream of logic. Since clouds are not solid, how can we get the sound of thunder? Thunder is not caused by any collision, but is caused by the heat of lightning. Lightning is hot, hotter than the surface of the sun. This burst of heat quickly expands air which causes the sound of thunder.
The best correlation would be a popping balloon. When the balloon breaks, the air rushes out and creates the sound we hear. But unlike a single pop, keep in mind that lightning is long and sound takes a while to make it to our ear. This fact along with the echoing off of the ground and other objects helps to bring us the rumble of thunder that we hear.