Here's why:

* Solids have tightly packed particles that are held together by strong forces. This structure gives them a fixed shape and volume. They resist changes in shape and volume.

* Liquids have particles that are closer together than gases, but they can move around more freely. This allows them to take the shape of their container, but they still have a defined volume.

* Gases have particles that are far apart and move randomly. They will expand to fill any container and have no fixed shape or volume.