To demonstrate the principle of buoyancy of warm air.
This experiment is to demonstrate the principle of buoyancy of warm air. Tie a small piece of cardboard to the end of the balloon's string. Adjust the size of the cardboard piece so that it is just heavy enough to keep the balloon from rising. Use a pair of scissors and start snipping off tiny pieces of cardboard so that you can adjust the balloon's weight until it hangs suspended in midair. The principle behind the experiment is because of the fact that cool air is heavier and denser than warm air and therefore has greater buoyancy. The balloon thus remains suspended at a point where it is too light to sink into denser air, which stays in the lower part of the room and is too heavy to rise into the warmer air above in the room.