The common plastic water bottle makes a useful container for demonstrating properties of gases and liquids. As typical examples, we know that "air" is a gas (made up of nitrogen, oxygen, water vapor, ozone, carbon dioxide, and several "trace" gases) and water is a liquid. We should also note that gases and liquids are both "fluids". That is, they can flow or change shape, rather than having a fixed shape like a solid.

So what happens when a water bottle is opened? Usually not much. What about with a bottle or can of shaken root beer though? In that case you're likely to get a messy explosion! The reasons are related to the properties of gases and liquids, especially as expressed through pressure and density.