A sponge is a porous material that can absorb and retain large amounts of water or other liquids. It is composed of a network of protein fibers called spongin, which are secreted by certain marine animals such as sponges. The spongin fibers are arranged in a way that creates pores or channels that allow water to flow through the sponge.

The composition of a sponge can vary depending on the type of sponge, but they typically contain a mixture of organic and inorganic materials, including proteins, carbohydrates, minerals, and pigments. The organic components are responsible for the sponge's elasticity and ability to absorb water, while the inorganic components provide structural support and rigidity.

Therefore, a sponge is a mixture of different materials and not a pure substance.