When lightning passes through air it leaves a vacuum, and the air rushing in to fill it makes the noise which we call thunder. We do not usually hear this until some time after the flash of lightning because light travels more than a million times faster than sound. When the thunder cloud is at a distance, the sound comes to us little by little and we then call it rolling thunder; but when the cloud is near the earth the sound comes in one great crash. You can generally tell how far off a thunder cloud is by noting how long the time is between the flash of lightning and the sound of the thunder. If you can count five as slowly as the tick of a clock between the two, you may be sure that the cloud is more than a mile away.