Empty a large, clear container (large pretzel or snack mix jugs). Fill part way with rubber bouncy balls. Use to demonstrate states of matter as follows:
Solids: barely shake the container, just enough to cause the balls to jiggle, but keep them in their place. The atoms/molecules in a solid possess kinetic energy, but not enough to remove any individual atoms/molecules from the group.
Liquids: shake the container a little harder, so the entire “clump” moves together. The atoms/molecules in a liquid posses more kinetic energy than those in a solid. Liquids can change shapes, but the atoms/molecules stay “attached” to one another.
Gas: shake the container even harder, so that the balls begin to bounce around the container, independent of one another. The atoms/molecules in a gas possess a large amount of kinetic energy. They will bounce around to fill the space they have available.
If you don't have a large container, or enough balls for a large container, you can make a smaller version (as seen above). It's probably not quite as effective as a demonstration as the larger version, but it works.