Final answer:
Cartilage is the substance covering the ends of bones in a joint, reducing friction and preventing them from scraping together. Synovial fluid produced in the joint also aids in reducing wear. The periosteum covers the bone's outer surface except at the joints, where articular cartilage is found.
Step-by-step explanation:
The ends of bones that help stop two bones from scraping together in a joint are covered by cartilage. This cartilage provides a smooth, almost glassy surface that reduces friction, much like how walking on ice has a low coefficient of friction. Joints also produce synovial fluid which further reduces friction and wear. When needed, due to damage or arthritis, an artificial joint may replace the natural joint, using materials with small coefficients of friction like stainless steel or titanium.
In addition to cartilage, the periosteum covers the outer surface of bones except where the epiphyses, or ends of the bones, meet to form joints. At the joints, the epiphyses are covered with articular cartilage. This layer of cartilage acts as a shock absorber and minimizes friction during movement.