In a paper mill, wood chips are softened using a process called "pulping." While your question mentions "big tank" and items like rocks, glass, stream, and ice, the actual process typically involves heating the wood chips in a large digester tank filled with water and chemical solutions. The main purpose of this is to break down lignin, cellulose, and hemicellulose in the wood chips, thereby softening and separating the fibers.
Common chemicals used in this process include sodium hydroxide (for kraft pulping) or sulfur dioxide (for sulfite pulping). Hot water and steam are also used to facilitate the cooking process. If you were referring to a specific method or alternative process, clarifying those would be helpful!