When thunder rumbles in the distance, there's a quick and simple way to figure out how close the lightning actually struck based on the speed of sound. After you see a flash of lightning, start counting. It takes about 5 seconds for the sound of thunder, which originates from lightning bolts, to travel 1 mile. If you can count to 10 between seeing the lightning and hearing the thunder, then the flash occurred about 2 miles away. This trick works because light travels much faster than sound.