The Roll Model® Therapy Balls (YTU OG’s, PLUS, ALPHA) are made of a natural rubber and will oxidize if exposed to moisture or prolonged sunlight. They are designed to soften with use. Direct sunlight, overexposure to air and prolonged moisture will ultimately dry the natural rubber out and they will lose their grip. It is best to keep them in a bag or drawer when not being used, but never keep them in a bag with wet clothes or towels as the moisture will also affect the balls. To clean these therapy balls, wipe with a lightly damp cloth, then remove all moisture with a dry cloth until the ball has regained its grip. Lack of use can lead to oxidation of the materials and could cause the balls to dry out leading to a slickness of the surface.