How does a bath sponge work?

A sponge is a loosely connected fiberous material that is primarily filled with empty space. Its that empty space that makes it so good at soaking up water. The gaps in the sponge soak up with water and then water tension holds the water from readily leaking out. If you compress an empty sponge you'll notice in reality the solid material of the sponge takes up very little space, but the way it is arranged on man made sponges (and the way the fibers arrange naturally on a living sponge) allow for a large amount of surface area for a small amount of material. hope that helps.

Although you will have some trouble seeing one, sponge is the remains of a living animal that grows on the bottom of the ocean, like coral. So rather than being invented, is was discovered, in ancient times. Modern sponges are plastic inflated with bubbles.