When a load is applied to a joint, fluid is squeezed out of the cartilage, allowing gliding with almost no resistance. Joints operate according to the principle of aquaplaning, with almost no friction and with constant cooling. For subsequent loading, water must be taken up and stored again. This is done by chondroitin and glucosamine. Chondroitin consists of chains of negatively charged disaccharides. They repel each other and as a result suck in water. Glucosamine stores this water in the cartilage matrix. It can absorb many times its own weight in water, and then store and release it again. It is supported in this by hyaluronic acid, which also contributes to the elasticity of the articular cartilage. It is also found in the joint capsules, tendons and ligaments of the joints. It forms a protective layer on the surface of the cartilage and is thixotropic. This means that, like ketchup, it changes its viscosity under pressure from a viscous to a thin fluid. Through this phenomenon, joint surfaces glide as one would slip on a banana skin. This also becomes suddenly fluid under pressure, and thus hyaluronic acid also acts as a shock absorber because it transfers pressure from non-elastic to elastic structures of the connective tissue, ligaments, tendons and muscles. Should the joint surfaces stick to each other when they touch, rips in the cartilage result (cartilage fissures). One doesn’t feel this however because the joint surfaces are not innervated. It only becomes apparent after some time as a rubbing sound, which is the first sign of degeneration. An audible cracking of the joints may even occur if the  joint surfaces are uneven. Vacuum phenomena are responsible for this as, during movement, fluid is squeezed out from the bumps and dents in the articular cartilage and then sucked in again. This is also observed in adolescents for whom it has no clinical significance. Stretching exercises and the administration of chondroitin and glucosamine, which significantly improve the sliding property of the joints, can reverse these signs of incipient cartilage damage.