The state of matter that keeps the same volume but changes shape when it's in a container is liquid.

Here's why:

* Liquids have a definite volume: This means they occupy a fixed amount of space. If you pour a liter of water into a glass, it will still be a liter of water, even if the shape of the glass changes.

* Liquids take the shape of their container: Unlike solids, liquids flow and conform to the shape of the container they are in.

Let me know if you have any other questions about states of matter!