Explanation:
Incompressibility refers to the property of a substance that does not change its volume when subjected to pressure. There are three states of matter: solid, liquid, and gas. Out of these three, solids are considered incompressible.

Reasons why solids are incompressible:
- Solids have a definite shape and volume, and the particles are closely packed together.
- The intermolecular distances in solids are small, and the particles are held together by strong intermolecular forces.
- When pressure is applied to a solid, the particles are unable to move closer together because of these strong intermolecular forces.
- Hence, solids do not change their volume when subjected to pressure, making them incompressible.

Why liquids and gases are compressible:
- In liquids, the particles are closely packed together, but they have more freedom of movement than in solids.
- When pressure is applied to a liquid, the particles can move closer together, reducing the volume of the liquid. Hence, liquids are compressible, but to a lesser extent than gases.
- In gases, the particles are far apart, and there are weak intermolecular forces between them.
- When pressure is applied to a gas, the particles can move closer together, reducing the volume of the gas. Hence, gases are highly compressible.

Conclusion:
In summary, solids are incompressible because of their strong intermolecular forces and closely packed particles. Liquids and gases are compressible because of their weaker intermolecular forces and greater freedom of movement.