The ends of bones at their joints are cushioned by cartilage, which is a type of firm, thick, slippery tissue that coats the ends of bones where they meet with other bones to form a joint. Cartilage helps reduce the friction of movement within a joint, absorbs shock, and supports structures in the body. It is made up of protein strands called collagen that form a tough, mesh-like framework filled with substances that hold water, much like a sponge. When weight is placed on cartilage, water is squeezed out of the mesh, and when weight is taken off, the water returns. Although cartilage is very strong, it can be damaged when a joint is injured.