The skeletal system has several functions including support, protection, shape and movement. It is made up of 206 bones classified into four groups. The skull protects the brain and gives shape to the head. The backbone protects the spinal cord and supports the weight of the body. The ribcage protects the heart and lungs while the pelvis protects the liver and kidneys. Cartilage cushions bones and reduces friction, found between bones and in places like the nose and windpipe. Ligaments are strong fibers that hold bones together at joints. Joints allow for movement and come in fixed, ball-and-socket, hinge, gliding and pivot varieties. Maintaining strong bones requires calcium and vitamin D. Common bone diseases include