An empty bottle (2 litre soft drink bottle)
A big bowl of hot water
A big bowl of cold water and ice
Blow the balloon to stretch it, then let the air out.
Stretch the mouth of the balloon and place it on the mouth of the empty bottle.
Place the bottle in the bowl of hot water and hold it in place for a few minutes. You will notice that the balloon expands.
Now, remove it from the bowl of hot water and place the bottle in the bowl containing cold water and ice. Here, you will notice that the balloon contracts.
How does this work?
The temperature inside the bottle increases when the bottle is placed in the bowl containing hot water. Therefore, the air inside it gets heated and expands. The molecules of the air move further apart occupying more space than before. This causes the balloon to stretch and expand. The balloon contracts when placed in the bowl containing cold water and ice because the temperature within the bottle reduces causing the air in it to cool down. This experiment shows that warm air occupies more space. This can be observed in hot air balloons.