A bone is a living structure made up of water, minerals such as calcium, and a tough protein, collagen. The outer part is very hard, but the inside is softer and spongy. Some bones contain a substance called marrow which makes red blood cells. Your delicate brain is well protected inside your cranium, the main part of your skull. This "brain box" is actually eight separate bones. As you get older, the bones gradually join together to make your skull stronger. The places where the bones join are visible as wiggly lines, called sutures.