Deuterium is a rare variation of hydrogen. For such a small object, it's incredibly informative. Here's how deuterium can let people track your your diet, your area, and your season.
Add a neutron to a hydrogen molecule and you get deuterium, a rare, heavy isotope. Deuterium makes up only one hundredth of a percent of the hydrogen out there, but it has a disproportionate influence on the sciences. The isotope, inside of water molecules, goes everywhere on the planet - including inside your body. The important thing is, it doesn't go there at the same rate.
A water molecule with an atom of deuterium is bulkier than other water molecules in its vicinity. This means that it resists evaporation and is inclined towards condensation. That leads to some interesting quirks. As summer goes on, and plants lose more and more of their water stores through evaporation, their deuterium supply is enriched. A famous study looked at doves in the Sonora desert. During the winter months these doves have a deuterium level that's the same as the ground water, but in the summer, their deuterium level goes way up. Why? It turns out that the doves stay alive by eating the saguaro cactus's flowers and fruit - and the saguaros had been upping their deuterium ratio as the lighter water evaporated away. It was possible to know what an animal was eating by examining the water in its body, even when the water was devoid of any other molecules that could give us a clue.
Deuterium's condensation pattern makes it more likely to condense once it is in the atmosphere. Certain forms of precipitation will carry more deuterium than others. Tracking the appearance of stable isotopes like deuterium allows scientists to figure out climate patterns, and to determine where the water in existing streams is coming from.
Finally, that deuterium isn't guaranteed to stay in water. Once deuterium enters your body, it can be ripped off the water molecule and end up anywhere, including your hair and nails. So someone stumbling on your corpse (it could happen) would be able not just to determine how much deuterium water you're drinking and eating now, but whether you had spent the last few years in a deuterium-rich area, or somewhere the isotope was scarce.
Image: William Herron