Gases don't have free surfaces because they have no fixed shape or volume. Unlike liquids and solids, which have definite boundaries, gases expand to fill any container they occupy.

Here's why:

* Weak intermolecular forces: The particles in a gas are far apart and have very weak attractive forces between them. This allows them to move freely and independently, constantly colliding with each other and the walls of their container.

* Random motion: Gas particles move in random directions and at high speeds. This constant motion prevents them from staying in a fixed position, thus they don't form a distinct surface like liquids or solids.

Analogy: Imagine a room full of bouncy balls. The balls represent gas particles. They bounce off each other and the walls, constantly changing their positions. There's no clear boundary or surface for the balls, just a random distribution throughout the room.

Key takeaway: The absence of free surfaces in gases is a direct consequence of their unique properties: weak intermolecular forces and random motion.