Here's why:

* Gases have weak intermolecular forces, meaning the molecules have a lot of freedom to move around. They collide frequently, but their spacing is much greater than in liquids or solids.

* Liquids have stronger intermolecular forces than gases. The molecules are closer together, but they can still move around.

* Solids have the strongest intermolecular forces. Molecules in solids are tightly packed and vibrate in fixed positions.

Think of it this way: imagine a room full of people.

* Gas: Everyone is running around, bumping into each other occasionally but with lots of space between them.

* Liquid: Everyone is still moving, but they're closer together and bumping into each other more often.

* Solid: Everyone is standing close together, barely able to move.