You can tell a lot about people by looking at their hair—and not just whether they brush, spray, or blow-dry. Scientists have found a way to use hair to figure out where a person is from and where that person has been. The finding could help solve crimes, among other useful applications. Water is central to the new technique. The liquid makes up more than half an adult human's body weight. Our bodies break water down into its parts—hydrogen and oxygen. But not all water is the same. Hydrogen and oxygen atoms can vary in how much they weigh. Different forms of a single element are called isotopes(核素). And depending on where you live, tap water contains unique proportions of the heavier and lighter isotopes of hydrogen and oxygen. Might hair record all this information? That's what scientists wondered. To find out, researchers collected hair from barbers in 65 cities in 18 states across the United States. The researchers assumed that the hair they collected came from people who lived in the area. Even though people drink a lot of bottled water these days, the scientists found that hair did reflect the concentrations of hydrogen and oxygen isotopes in local tap water. That's probably because people usually cook their food in the local water. What's more, most of the other liquids we drink contain large amounts of water that also come from sources within their region. Scientists already knew how the composition of water varies throughout the country. By combining that information with the results, they predicted the composition of hair in people from different regions. The new technique can't point to exactly where a person is from, because similar types of water appear in different regions that span(跨越)a broad area. But authorities can now use the information to analyze hair samples from criminals or crime victims and narrow their search for clues. Based on the finding, one research offers this advice:"If you're a criminal, shave."