“Bones support your body and allow you to move. They protect your brain, heart, and other organs from injury.

Bone is a living, growing tissue. It is made mostly of two materials: collagen (KOL-uh-juhn), a protein that provides a soft framework, and calcium (KAL-see-uhm), a mineral that adds strength and hardness. This combination makes bone strong and flexible enough to hold up under stress.

Bone releases calcium and other minerals into the body when you need them for other uses.

Think of bones like a bank, you deposit and withdraw bone tissue.  You deposit in childhood and teenage years when the bones grow.  Than older bones withdraw causing bones to become larger, heavier, and denser.”

NIH National Institute of Arthritis and Musculoskeletal and Skin Disease