Key trace minerals greatly strengthen teeth: The outer layers of teeth are made up of nanowires of enamel that are prone to decay. Gordon et al. analyzed the composition of tooth enamel from a variety of rodents at the nanometer scale (see the Perspective by Politi). In regular and pigmented enamel, which contain different trace elements at varying boundary regions, two intergranular phases—magnesium amorphous calcium phosphate or a mixed-phase iron oxide—control the rates of enamel demineralization. This suggests that there may be alternative options to fluoridation for strengthening teeth against decay. Science , this issue p. 746 ; see also p. 712