Open the grocery bags and poke a hole in the center of each bag's
bottom. Cut two strings of equal length and tie a knot at one end of
each string. Insert the strings through the holes in the bags and tie
the loose end exactly 1" from each end of the yardstick. Set the
block of wood on the corner of a desk so the yardstick will move
freely without obstructions. Be careful to make sure the yardstick is
perfectly balanced. Place the incandescent lamp under one of the
grocery bags and turn it on. Watch what happens.
The incandescent bulb will produce heat. The warmer air stream
will replace the colder air that is in the bag. As the colder air is
replaced with the warmer air, the bag becomes lighter and will rise.
This demonstrates that warm air is less dense than cold air, and that
cold air of equal volume will weigh less than warm air. Since cold
air is heavier than warm air, cold air will fall pushing the warmer
air up.