Take the balloon and stretch it gently over the lip of the soda bottle. Place the bottle into the pan of hot water and let it stand for a few minutes. To reverse the outcome, place the bottle into the pan of ice water and let it stand for a few minutes.
This activity demonstrates that the air inside the soda bottle expands when heated causing the air molecules to move faster and get farther apart. This is what causes the balloon to inflate. Warm air is less dense than cold. Since warm air takes up more space than the same amount of cold air it makes the balloon inflate.