Water molecules aren't as permanent as atoms. They're an association of atoms, which are around more or less forever, but if a water molecule breaks apart and the atoms go their separate ways, that’s the end of that particular water molecule.

The question remains: Did a dinosaur or a Neanderthal ever drink the same water we’re drinking? Stager says “by the laws of chance, it’s possible, ‘cause there’s just so many of them out there. Maybe some of them were hiding away in the depths of the ocean forever and ever.” (One example of this "hidden" water comes from Soudan, Minn., where ancient water was discovered at the bottom of a mine in 2011.)

There are several things that could have happened to those molecules. Water exists as a liquid and a solid (ice), but it also very commonly exists as a gas in the atmosphere. In the upper part of the atmosphere, powerful radiation from the sun and cosmic rays from space will hit the gaseous water molecules, and break them apart. So, “the sun is always breaking a bunch of them.”

“If they circulate through the ocean and then evaporate into the air they’re at risk. So over time a lot of them are lost from that, but then the really big danger for a water molecule, if you’re ever reincarnated as one, you better watch out, don’t go near plants, because they shatter water molecules—that’s where our oxygen gas comes from.”

When plants make oxygen gas, they get those atoms from water molecules, splitting them apart with help from the sun’s energy—that’s how photosynthesis works. Stager says scientists have done the math, and between plants, algae, and some forms of bacteria, which also perform photosynthesis, “since we’ve had these life forms for billions of years, all the water molecules on the earth must have been shattered and recycled dozens of times through the history of life by these living things.”

We’re not making more atoms, but they get moved around so “what was in a water molecule for a million years suddenly becomes part of your hair, or part of a leaf or something like that.”

Most of the water in our body is recycled from water we’ve drunk or eaten—but a small amount is made from scratch. That’s something most living things do, when we breathe oxygen in, our bodies process it, and it becomes something called “metabolic water.” So while we think of water as ancient, some water molecules in the world are younger than we are.