States of Matter (Book)
Solids, Liquids, Gases, and Plasmas
Water vapor is an example of a gas. A gas is matter that has neither a fixed volume nor a fixed shape. Instead, a gas takes both the volume and the shape of its container. It spreads out to take up all available space. You can see an example in Figure.
When you add air to a bicycle tire, you add it only through one tiny opening. But the air immediately spreads out to fill the whole tire.