Although osteoporosis has long been thought of as a condition primarily affecting bone quantity (bone mineral density), mounting evidence suggests that bone produced in older adults and osteoporosis patients differs in the quality of the bone material as well. Not only is there less bone, but the bone that is present is not as fracture-resistant as normal bone. Bone is a composite material made up of tiny calcium phosphate mineral crystallites embedded in collagen fibers. It is well established that, aside from its composite nature, this unique material's mechanical properties arise from its hierarchical three-dimensional organization. Of particular importance to the mechanical properties of a composite material are the interactions between the components. We have used a powerful technique, solid-state nuclear magnetic resonance (NMR) to explore that interface. We will present evidence from ¹H-³¹P and ¹H-¹³C dipolar coupling NMR that an organized water layer mediates the interaction between the mineral crystallites and the collagen fibers. This organized water layer may play a vital role in the toughness of bone, providing a sacrificial layer that can absorb energy without damage to the bone structure. We observed an irreversible loss of this water layer upon dehydration of the bone samples, leading to a brittle material. A similar loss in the bones of osteoporotic patients may partially account for the observed poor bone quality.