* Mass: Gases are made up of molecules, and molecules have mass. Therefore, any given amount of gas will have a certain mass.

* Volume: Gases occupy space, just like solids and liquids. They expand to fill the container they are in, meaning they have a volume.

It's important to remember that:

* The density of gases is usually much lower than solids and liquids. This is because the molecules in a gas are much further apart than in a solid or liquid.

* The volume of a gas can change easily with changes in temperature and pressure.