Here's why:

* Solids have a fixed shape and volume because their particles are tightly packed together in a regular, repeating arrangement. The strong forces of attraction between the particles keep them from moving freely.

* Liquids have a fixed volume but take the shape of their container. The particles are close together but can move around each other.

* Gases have no fixed shape or volume, and their particles are far apart and move freely.

Let me know if you have any more questions!