Here's why:
* Solids have a fixed shape and volume. Particles in a solid are tightly packed together and vibrate in place, but they don't have enough energy to move freely.
* Liquids have a fixed volume but can change shape. Particles in a liquid are closer together than in a gas but have more freedom to move around.
* Gases have no fixed shape or volume. Particles in a gas are far apart and move freely, colliding with each other and the walls of their container.
So, the particles in a solid are the least mobile, resulting in the least amount of motion.
