What causes thunder?

Thunder is most often caused by lightening.  Lighting is
very hot and it rapidly heats the air around it as it strikes.  This rapid heat causes
the air to expand.  The sound of the air expanding is what we call thunder.  It is
similar to the creation of sound during an explosion.  Since light travels faster than
sound, we can calculate the distance of a lighting strike by counting the seconds
between the lightning and the thunder.  We count seconds because thunder isn't generally
heard far enough away to count the elapsed time in minutes.  In general, each second
between the lightning and the thunder is about a mile.  There is a precise number, but
that is a more difficult calculation.