A version of this article originally appeared on The Conversation.

You probably do it. It might be ingrained from when you were a kid, and now it’s almost automatic. You see the flash of lightning – and you immediately start counting the seconds till it thunders.

But does counting really get you a good estimate for how far away the lightning is? Is this one of those old wives’ tales, or is it actually based on science? In this case, we have physics to thank for this quick and easy – an