Beauty is only skin deep. And the beauty of shiny white teeth is even less deep. Because a new study shows that fluoride forms a thinner protective shield than experts thought it did. The results appear in the surface science journal Langmuir. American consumers spend more than fifty billion dollars a year fighting cavities. When we realized that fluoride could help, we put it in our drinking water, our toothpaste and our mouthwash. But how does fluoride work its magic? Many figured that fluoride chemically reacts with the main mineral in enamel to form a thick, decayresistant veneer. But the latest research kicks that idea in the teeth. Scientists in Germany used x-ray photoelectron spectroscopy to get a detailed image of the surface of a treated tooth. And they found that the protective covering is actually only 6 centimeters thick. That's about a fifteen-thousandth as thick as a sheet of paper. And a layer that skimpy would likely be worn away by ordinary chewing.