Question

Explain why a sponge is called a solid even though it can be compressed

Answer 1

Because it has a definite shape and definite volume and also most importantly it doesn't satisfy the criteria of other states of matter like it's not like liquid that it would take shape of its container or not as gas whish has no shape and volume.
One important point to be considered is that not all solids are hard. Leaf, paper,wax all are solids but they aren't hard same is the case with sponges.

Answer 2

A sponge is said a solid even it can compressed because its particals are very close. It can compresed and after releasing it comes in its real age.