Why does thunder rumble?

A lightning bolt can heat the air around it to about 30,000 °C, roughly five times hotter than the surface of the Sun, in a fraction of a second. The superheated air expands violently, producing a shock wave that we hear as thunder. A nearby strike sounds like a sharp crack, but a bolt can be several kilometres long, so sound from its nearer and farther sections arrives at slightly different times, stretching into a long rumble. Hills, buildings and layers of air also echo and bend the sound. Because light is almost instant and sound travels about 343 metres per second, counting the seconds between flash and thunder tells you how far away the storm is.