Tooth enamel has a tough job. Surrounded by hostile mouth bacteria that produce corrosive acid, enamel must withstand the wear and tear of grinding food. Scientists have known that well-ordered crystalline hydroxyapatite nanowires help enamel resist cracking and form the bulk of the material. But a new study suggests how amorphous components—present in only small amounts—can deliver significant hardness and corrosion resistance to enamel (Science 2015, DOI: 10.1126/science.1258950). Northwestern University’s Derk Joester and colleagues used a potpourri of physical techniques to peer at the complex structure of pearly rabbit, mouse, and rat enamel as well as stained beaver enamel, which is harder and more resistant to acid attack. They found that the amorphous parts of normal enamel are composed primarily of magnesium-substituted amorphous calcium phosphate (Mg-ACP). Beaver teeth get their added hardness and their stain from the replacement of Mg-ACP with iron-containing mineral and phosphates. This study may eventually lead to enamel-strengthening strategies that prevent or minimize cavities, the authors note.