The sentence illustrates the conservation of mass by stating that although the books and magazines were destroyed in the fire, their mass was not lost. Instead, the materials transformed into ash and water. The principle of conservation of mass asserts that mass cannot be created or destroyed in a chemical reaction; it can only change forms. In this scenario, the total mass of the original materials (books and magazines) is equal to the mass of the products (ash and water) after the fire, demonstrating that the total mass remains constant.