You have probably cracked open a soda before to see the liquid fizz right up out of the bottle, creating a huge mess. Why does that happen? It has to do with the carbon dioxide gas that is added to the liquid to make it fizzy. Opening the bottle releases the built-up pressure by the gas inside, causing the gas-liquid mixture to rush out the bottle. In this activity, you will demonstrate with the help of air- and water-filled balloons how a gas changes volume depending on its pressure.