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 in place.
* Liquids have a fixed volume but can take the shape of their container. Their particles are more loosely packed than solids and can move around each other.
* Gases have no fixed shape or volume and can expand to fill their container. Their particles are far apart and move freely at high speeds.
So, while particles in liquids and gases do move, the movement in solids is significantly slower and more restricted.
