If you were an alien race studying human taste, you might expect carbonated water to be downright repellent."We tried to get mice to drink carbonated water--and they don't. They don't like it," says Nicholas Ryba, a senior investigator at the National Institute of Dental and Craniofacial Research. That, Ryba says, indicates there's not a chemical process that naturally draws all mammals to carbonated water. Ditto for horses and other animals, says Bruce Bryant, a senior research associate at the Monell Center, a nonprofit research institute for taste and smell. Animals have several CO2-detecting systems, possibly a gift from evolution for detecting not-so-great stuff that emits carbon dioxide, like rotting food.