Science Matters is an awesome blog with fun science experiments and demonstrations for kids, to keep them engaged in lessons. This demonstration on matter (atoms and molecules in the various states) using bouncy balls is simple and teaches kids about kinetic energy. Click here to view the full demonstration!
Afterwards, have kids glue Cheerios to show molecules in each state. Visit Foreman's Fourth Grade to see activities her class did, and what they included in their Cheerios and Matter foldable.