Here's why:

* State of Matter: The three main states of matter are solid, liquid, and gas.

* Solids have a fixed shape and volume.

* Liquids have a fixed volume but take the shape of their container.

* Gases have no fixed shape or volume and expand to fill their container.

* Air's Properties: Air is a mixture of gases, primarily nitrogen and oxygen. These gases spread out to fill any space they occupy. They don't have a fixed shape or volume.

Think of it this way:

* You can hold a solid rock in your hand.

* You can pour a liquid like water into a glass.

* You can't hold air in your hand, and it fills the entire room.

Let me know if you have any other questions!