Here's why:

* Solids have a fixed shape and volume. The particles are tightly packed together and vibrate in place, but they don't have enough energy to move freely.

* Liquids have a fixed volume but not a fixed shape. The particles are closer together than in a gas, but they can move around each other, giving liquids their fluidity.

* Gases have no fixed shape or volume. The particles are far apart and move freely, colliding with each other and the walls of their container.

So, the slower the particles move, the more tightly packed they are, and the more rigid the state of matter.