Here's why:

* Large distances between particles: Gas particles are constantly moving and have a lot of space between them. Think of it like a room full of people compared to a crowded elevator.

* Weak intermolecular forces: The forces of attraction between gas particles are very weak compared to liquids and solids. This allows them to move freely and spread out.

* High compressibility: Gases can be compressed easily because there's so much empty space between the particles.

This is in contrast to liquids and solids, where particles are much closer together and have stronger intermolecular forces.