Bone Strength Bone is both strong and lightweight. Bones can absorb more force without breaking than concrete or granite rock can. Yet bones weigh much less than those materials. In fact, only about 20 percent of an average adult's body weight is bone. Bone feels as hard as a rock because it is made of tightly packed minerals-mainly phosphorus and calcium. Bone Growth Because bones are alive, they form new bone tissuw as you grow. Your bones are growing longer now, making you taller. Even after you are fully grown, bone tissue continues to form. For example, every time you play soccer or basketball, some of your bones absorb the force of your weight. They respond by making new bone tissue. New bone tissue also forms when a bone breaks. Bone Development Because bon Know your bones! Here are some fascinating facts you may not know about your bones: