The Harold and the Purple Crayon couch gag is the sixteenth couch gag of Season 21.
Harold, from the children's book Harold and the Purple Crayon, draws the Simpson living room with his purple crayon. After he finishes, the Simpson family runs into the room and sit on the couch as usual. Homer asks Harold to draw him a can of beer, and Harold agrees to draw it for him. While Homer drinks the beer, he pats Harold on the head, as a way for saying thanks.