The word "汗牛充栋" is formed by combining two vivid images: "汗牛", which literally means "sweating oxen" from transporting heavy loads, and "充栋", which means "filling a house to the beams". Together, these components directly depict a scene where books are so numerous that they cause oxen to sweat under their weight and completely occupy a building, thus conveying the idea of an extensive collection of writings through this concrete, literal juxtaposition.