Here's why:

* Liquids: The molecules in a liquid are close together but can move around freely. This allows them to flow and take the shape of the bottom of their container.

* Gases: Gas molecules are very far apart and move freely in all directions. This allows them to expand to fill the entire volume of their container.

Solids have a fixed shape and don't take the shape of their container. Their molecules are tightly packed and vibrate in place.