Fact for September 2, 2008
This may seem trivial to some, but, surprisingly a lot of people do not realize this: thunder is the sound of lightning. It is the sonic boom created by a lightning flash ionizing the air around it. Some lightning can seem to occur without thunder but this simply means it was likely a weak flash and the sound was subsonic or too weak to be heard (dissipated perhaps). Thunder can also seem to appear without lightning, but this is likely cloud-to-cloud lightning whose light flash is shielded by the thick clouds, or perhaps it is too far away.
This is why you can roughly figure out where a storm is by counting the seconds between a lightning flash and the sound of thunder. You’re essentially measuring the difference between the speed of sound and the speed of light.