Cut out the propeller from the tagboard and cover it with aluminum
foil. Make a cylinder from the remaining tagboard to wrap around the
jar. Suspend the propeller above the cylinder using the needle and
the pipe cleaner so it spins freely. Place the cylinder over the jar
of warm water.
The warm water will create an air stream that flows out from the
cylinder. This demonstrates that warm air molecules are less dense
than the cold air around the cylinder making the propeller to turn
freely. The propeller will continue to spin until the water has
cooled.