Here's why:

* Shape: Gas molecules are constantly moving and colliding with each other and the walls of their container. They don't have a fixed shape like solids or liquids. Instead, they take on the shape of the container they are in.

* Volume: Gas molecules are spread far apart and move freely. They expand to fill the entire volume of their container, no matter how large or small it is.

Example:

Imagine a balloon filled with air. The air inside the balloon takes on the shape of the balloon. If you squeeze the balloon, the air inside will compress and take on a smaller volume. If you let go, the air will expand again to fill the entire space of the balloon.

Key takeaway:

Gases are highly compressible and easily change their shape and volume to match their container. This is a fundamental property of gases that distinguishes them from solids and liquids.