Invisible germs called bacteria live in your mouth all the time. Some of these bacteria form a sticky material called plaque on the surface of the teeth. When you put sugar in your mouth, the bacteria in the plaque gobble up the sweet stuff and turn it into acids. These acids are powerful enough to dissolve the hard enamel that covers your teeth. That’s how cavities get started. If you don’t eat much sugar, the bacteria can’t produce as much of the acid that eats away enamel.