You need some ping pong balls, tennis balls or other smallish balls. You also need some blutac or other sticky stuff.

To represent a solid, get the balls and stick them together with blutac. They need to be stuck together in a regular way.

To make a liquid, take the blutac off and put all of the balls on the floor of a hall. Put a rope around them. The balls can now move, but they all keep together.

Now take the rope away and throw the balls all over the hall. Now you have a model of a gas.