Here's why:
* Solids have a fixed shape and volume because their particles are tightly packed together in a regular, ordered arrangement. They can only vibrate slightly around their fixed positions.
* Liquids have a fixed volume but not a fixed shape. Their particles are closer together than in gases but can move around each other, allowing the liquid to flow.
* Gases have no fixed shape or volume. Their particles are far apart and move freely and randomly, colliding with each other and the walls of their container.
So, only in a solid state are particles restricted to vibrating in place.
