The raw materials used to make them. Smooth-On never uses "off spec" or "waste stream" polyols that are often used to formulate foams for making products like furniture, shoes and other commodities. Smooth-On also uses filtered water in the production of its foams. As a result, Smooth-On foams have higher physical and performance properties than waste stream foams. With several densities to choose from, Smooth-On foams are premium formulations that are consistent from batch to batch ensuring the user a more predictable outcome every time.