Here's why:

* Solids have a fixed shape and volume. The molecules are tightly packed together in a regular, repeating arrangement called a crystal lattice. They can only vibrate in place, not move freely.

* Liquids have a fixed volume but a changing shape. The molecules are closer together than in gases, but they can move around each other.

* Gases have no fixed shape or volume. The molecules are far apart and move around freely.