The term 'knock on ___' can be completed with the solutions WOOD, RAP, and rapat. WOOD is a hard, fibrous material that is commonly used in construction and furniture making. RAP is a type of music that originated in the United States in the 1970s and is characterized by its use of rhythmic spoken or chanted lyrics. Rapat is a term used in Indonesian culture to describe a traditional form of knocking on a door as a way of announcing one's presence.