Pound for pound, human bone is stronger than steel.

Bone isn’t strong in an absolute sense — a steel beam can bear far more total load than a human femur ever could. But relative to its weight, bone has a higher strength-to-weight ratio than steel, thanks to a composite structure of flexible collagen fibers reinforced with rigid mineral crystals of calcium phosphate.

That combination lets bone resist both compression and a fair amount of bending without being brittle, a balance few engineered materials achieve as efficiently. It’s also a living, self-repairing material: bone tissue is constantly being broken down and rebuilt throughout life, letting it adapt its density and internal structure in response to the loads it regularly carries.