A BSTRACT Introduction: How fluoride (F – ) protects dental enamel from caries is here conveyed to dental health-care providers by making simplifying approximations that accurately convey the essential principles, without obscuring them in a myriad of qualifications. Materials and Methods: We approximate that dental enamel is composed of calcium hydroxyapatite (HAP), a sparingly soluble ionic solid with the chemical formula Ca 10 (PO 4 ) 6 (OH) 2 . Results: The electrostatic forces binding ionic solids together are described by Coulomb’s law, which shows that attractions between opposite charges increase greatly as their separation decreases. Relatively large phosphate ions (PO 4 3– ) dominate the structure of HAP, which approximates a hexagonal close-packed structure. The smaller Ca 2+ and OH – ions fit into the small spaces (interstices) between phosphates, slightly expanding the close-packed structure. F – ions are smaller than OH – ions, so substituting F – for OH – allows packing the same number of ions into a smaller volume, increasing their forces of attraction. Dental decay results from tipping the solubility equilibrium Ca 10 (PO 4 ) 6 (OH) 2 (s) ⇔ 10Ca 2+ (aq) + 6PO 4 2– (aq) + 2OH – (aq) toward dissolution. HAP dissolves when the product of its ion concentrations, [Ca 2+ ] 10 ×[PO 4 3– ] 6 ×[OH – ] 2 , falls below the solubility product constant (Ksp) for HAP. Conclusion: Because of its more compact crystal structure, the Ksp for fluorapatite (FAP) is lower than the Ksp for HAP, so its ion product, [Ca 2+ ] 10 ×[PO 4 3– ] 6 ×[F – ] 2 , must fall further before demineralization can occur. Lowering the pH of the fluid surrounding enamel greatly reduces [PO 4 3– ] (lowering the ion products of HAP and FAP equally), but [OH – ] falls much more rapidly than [F – ], so FAP better resists acid attack.