sciencehabit writes with an intriguing story about the potential of figuring out where people have been by examining their hair: "That's because water molecules differ slightly in their isotope ratios depending on the minerals at their source. Researchers found that water samples from 33 cities across the United State could be reliably traced back to their origin based on their isotope ratios. And because the human body breaks down water's constituent atoms of hydrogen and oxygen to construct the proteins that make hair cells, those cells can preserve the record of a person's travels. Such information could help prosecutors place a suspect at the scene of a crime, or prove the innocence of the accused."
Or frame someone by slipping them water from every country on the terrorist watchlist.