What is the substance that makes the cartilage at the end of the bones slippery?

The substance that makes the cartilage at the end of the bones slippery is called synovial fluid. This viscous fluid is produced by the synovial membrane and serves to lubricate the joints, reduce friction during movement, and provide nourishment to the cartilage. The combination of cartilage and synovial fluid allows for smooth and pain-free joint motion.