Your breath has been shared by many before you.
You might have heard the old story that we -- as in you and I and everyone on the planet right this second -- are breathing in the same molecules that Julius Caesar breathed. Is it true?
The answer is a strong ... could be? Some scientists argue that molecules are constantly shifting and rearranging, but atoms are a different story. Every atom on the planet has pretty much been here forever, minus some asteroid impacts. So let's say Aristotle breathed in oxygen back in the day. Through the years, that oxygen atom could've hooked up with carbon, which helped make a cellulose molecule, which through photosynthesis could've been released back into the atmosphere for you to breathe. It's way unlikely, say some scientists, that this recycling of atoms resulted in everybody breathing everyone else's molecules [source: St. Maurice].
But theoretical physicist Lawrence Krauss supports the theory that the molecules we breathe are redistributed evenly in our atmosphere within a number of centuries. And if that's the case, he argues for a mathematical probability that more than 99 out of 100 of our breaths will contain molecules from Caesar [source: Krauss]! And Cleopatra! And the Queen of England, the band Queen and so on.
At the very least, every single breath you take has been associated with another living organism. So go ahead and wow the kiddies by claiming their breath might contain atoms once belonging to the tusk of an elephant, a tree limb or their kitty cat's paw.