The five second rule allows you to determine how far away a lightning strike, or pretty much any event that you can both see and hear, is.

Sound travels about one mile every five seconds, so if you see some lightning, and start counting seconds, then hear thunder, you can estimate how many miles away the lightning struck by dividing the number of seconds by five. If you never hear the thunder, the strike was probably too far away to matter anyway.

This can also be used to find out how far away fireworks are when they go off. See the boom, start counting, hear the boom, divide. It's that simple.