Here's why:

* Gases have very weak intermolecular forces between their particles. This means the particles have a lot of freedom to move around, colliding with each other and the walls of their container.

* Liquids have stronger intermolecular forces than gases, so their particles move more slowly and are closer together.

* Solids have the strongest intermolecular forces, so their particles are tightly packed and vibrate in place.

The faster the particles move, the higher the temperature of the substance.