Here's why:

* Solid: Salt has a fixed shape and volume. It doesn't flow like a liquid or expand to fill a container like a gas.

* Liquid: Liquids have a fixed volume but take the shape of their container. Salt doesn't do this.

* Gas: Gases have no fixed shape or volume and expand to fill their container. Salt doesn't behave like this either.

Salt can be melted into a liquid if heated to a high enough temperature, but at room temperature, it's a solid.