A sponge has holes and soaks up liquid. One kind of sponge is a water-dwelling animal with a soft body and an asymmetrical shape. But the kind of sponge you use to clean the kitchen is usually man-made.
The sponge in the kitchen or bath looks a bit like the kind that grows in the ocean, porous and soft. Originally, those sponges were made from dried sea sponge, but it's more common these days for them to be manufactured. If you absorb knowledge or information easily, people may call you a sponge. And if you live off someone else's generosity, taking handouts and giving nothing in return, you sponge off of them.