Explain:
$(a)$ Sponge,though compressible,is a solid.
$(b)$ Gases completely fill the vessel in which they are kept.

(N/A) sponge has minute holes in which air is trapped. On applying pressure or force,the air is expelled out and the sponge gets compressed. On releasing the pressure,the sponge regains its shape. Thus,a sponge is classified as a solid.
$(b)$ $(i)$ In the gaseous state,particles have high kinetic energy,which allows them to move freely and maintain large spaces between them.
$(ii)$ Therefore,the particles occupy the entire space available. Hence,gases completely fill the vessel in which they are kept.