Although they vary in what they are made of, all skateboard bearings are standardized to fit into skateboard wheels. They contain lubricated metallic balls and are the key to a smooth ride. To maintain this smooth ride, it is important to keep your bearings clean (avoid water and sand while skateboarding). Some may choose to clean their bearings from time to time, but do not do so with WD-40! You will ruin your bearings. Only special bearing lubricant should be used to clean them.