The load-bearing material in bone is a fascinating nanocomposite of collagen, a fibrous protein, and bioapatite, a calcium phosphate, in a 50:40 volume ratio, with 10% water. The interface between the ~2.8-nm thick apatite nanocrystals and the collagen matrix is crucial for the mechanical properties of bone. Modern 31 P, 13 C, and 1 H nuclear magnetic resonance experiments have provided detailed information on the nanocrystal composition and thickness, the apatite surface composition, water layers at the apatite-collagen interface, and apatite-binding sites of collagen. The COO - groups of most glutamates and 1/4 of the C-OH moieties of hydroxyprolines in collagen are near the interface, at a ~0.5 nm distance from phosphorus in apatite. A layer of viscous H2O detected at the organic-inorganic interface may act as "glue" between apatite and collagen, providing continuous bonding. This view is supported by large-amplitude motions of the interfacial C-OH groups of hydroxyprolines, which make their specific binding to apatite surface sites unlikely. The new findings elucidate the role of water for the mechanical properties of bone and provide guidance for the synthesis of bone-mimetic materials.