As air is warmed, the particles (molecules) it is made up of spread out. This makes the air lighter (less dense), and it rises. As the air in the tissue balloon is warmed it expands and rises. The rising warm air fills the balloon and causes it to rise, also.

The student is able to communicate that the hair dryer warmed the air inside the tissue balloon making the air lighter. The lighter air rose and filled the balloon causing it to rise, also. The student is able to communicate that the warm air caused the balloon to rise. The student is able to communicate that the hair dryer made the balloon rise. The student is able to communicate that the balloon rose.