They rotate from station to station completing each activity. Crumple up a paper towel and jam it into the bottom of a clear plastic cup. Hold the glass upside down and, keeping it straight, push it completely under the water. The paper towel does not get wet. Air is everywhere and takes up space.
Air takes up space in the glass and acts as an invisible wall between the water and the paper.
The envelope of a hot-air balloon contains air. Place the pan of water on the hot plate. Place the bottle in the water. Heat the water and observe the balloon. Place bottle on table, and observe the balloon.