What is the term for when a sponge fills with water?

Absorption is the process through which a sponge takes in liquid, allowing it to fill with water. In this context, absorption specifically refers to the phenomenon where a material, such as a sponge, takes in a liquid through its porous structure. The molecular structure of the sponge facilitates this process, enabling the water molecules to be drawn into the sponge's open spaces.

Expansion, while it implies a change in size or volume, is not specific to the process of water being taken into the sponge. It could refer to any number of materials or situations. Permeation relates to the process of a liquid or gas passing through a material, but it does not encapsulate the action of a sponge actively absorbing water. Adsorption involves the accumulation of molecules at the surface of a solid or liquid, which is different from the internal absorption process where the water fills the sponge. Thus, absorption is the most accurate term to describe the filling of a sponge with water.