Here's why:

* Gases are compressible: The molecules in a gas are far apart and have weak attractions to each other. This means you can easily push them closer together, reducing the volume they occupy.

* Gases expand to fill their container: If you put a gas in a container, it will spread out to fill the entire space, taking on the shape and volume of the container.

Think of it this way:

Imagine you have a balloon filled with air. If you squeeze the balloon, you're decreasing the volume of the air inside. If you release the pressure, the balloon will expand to fill the space again.

This is because gases have no fixed volume, they simply adapt to the space they are in.