‘The hard stuff: calcium, cells, bones, and cancer’ considers calcium and the hormone systems that regulate it. These hormones keep the correct balance between the highly reactive Ca2+ ions, which are essential for many life processes, and the precipitating salts like carbonates and phosphates. Controlled precipitation builds bones that defy the pull of gravity and hard materials that protect against predators. Bone formation and recycling take energy, as well as calcium and phosphorus, and all aspects of skeletal development are affected by the hormones of growth and metabolism. There are connections between bone development and appetite which work through serotonin, leptin, insulin, and hormones produced by the gut, in addition to calcitonin.