Here's why:

* Liquids have a definite volume but take the shape of their container. The soda in the can has a fixed amount, but it conforms to the shape of the can.

* Solids have a definite shape and volume. The can itself is a solid.

* Gases have no definite shape or volume and will expand to fill their container. While carbon dioxide gas is dissolved in the soda, it's not the primary component that determines the state of matter.