The bone is made up of calcium. Calcium along with collagen helps to make the bones very strong, like concrete. Bones have a very high tensile strength, that is why we can put the pressure of our body weight on them without the bones being broken immediately. The structure of the bone also helps since the shaft is made up of compact bone which is very good at resisting tension. To help with the compressional strength of the bones, the extremities are made of cancellous material which is very good at fighting compression. All in all, the material and engineering of the bone allow it to be rigid, yet flexible, making them hard to break and good at their function of support and protection.