This past unit in science we covered states of matter and how they change. Students have to understand how molecules move at each phase and the energy involved. There are a ton of demos that show the phase changes and this is one of my favorites. All that you need is a large flask, water, a water balloon, a hot plate and tongs. I have my students draw diagrams of how the molecules are arranged and moving at each phase and the transitions inbetween. They also have to determine if heat energy is being added or taken away in each change. Even in the digital age, I think students benefit from simple pencil and paper drawings. The drawings are really models that explain the scientific phenomena. When the balloon gets pushed into the flask, it is a very dramatic demonstration of a liquid taking up less space than a gas.