The human thigh bone is 4 times stronger than concrete and stronger than a steel rod of the same weight. Our leg bones give support to and bear our entire body weight, so they have to be really strong. The lattice structure inside the bones makes them tough, yet light enough for us to move around easily. However, a big force at the wrong angle can break the bone.
Bones are under constant stress, so parts are always being broken down and replaced. One of the main components of your bone is calcium. If you don't have enough calcium in your diet, it can cause your bones to break more easily. The extra stress put on your bones during exercise makes them stronger. Astronauts who have been floating around in space have weaker bones due to lack of stress.