$(a)$ Compare a block of wood,water,and air on the basis of the following:
$(i)$ Compressibility
$(ii)$ Particle motion
$(iii)$ Rigidity
$(b)$ Account for the following:
$(i)$ Gases exert pressure on the walls of the container.
$(ii)$ Liquids can be called fluids.

(N/A) The comparison is as follows:

Property	Block of wood	Water	Air
$(i)$ Compressibility	Negligible	Very low	High
$(ii)$ Particle motion	Least (vibratory only)	More than wood	Very high (random)
$(iii)$ Rigidity	Rigid	Fluid	Fluid

$(b)$ $(i)$ Gases exert pressure on the walls of the container because gas particles are in constant random motion at high speeds. When they collide with the walls of the container,they exert a force per unit area,which is perceived as pressure.
$(ii)$ Liquids are called fluids because they have the ability to flow and change their shape according to the container they are placed in,which is a characteristic property of fluids.