Question

a sponge can be compressed easily so why do we call it a solid

Answer 1

Because it is opaque and light cannot pss through them

Answer 2

Sponge has small pores in it , and air gets trapped in these pores , so when we compress the sponge the air inside it is expelled out , after air is expelled out it comes back to its original shape, that is why we call
sponge as solid.