Here's why:
* Solids: The molecules in a solid are tightly packed and held together by strong forces. This gives them a fixed shape and volume.
* Liquids: While the molecules in a liquid are not as tightly packed as in a solid, they still have strong enough attractions to maintain a definite volume. Liquids can flow and take the shape of their container, but their volume remains constant.
Gases, on the other hand, have neither a definite shape nor a definite volume. They expand to fill the container they are in.
