The term 'Smooth as __' can be completed with either the 5-letter word 'GLASS' or the 4-letter word 'SILK'. GLASS is a material made from sand, soda ash, and limestone that is heated to very high temperatures until it melts and forms a transparent solid. SILK is a natural protein fiber produced by certain insects, most notably silkworms, which is used to make fabric. Both GLASS and SILK are known for their smooth texture, making them ideal answers for the crossword clue.