Here's why:

* Matter is Conserved: The law of conservation of mass states that matter cannot be created or destroyed, only transformed. This means that the total mass of a system remains constant, even if it changes state (solid, liquid, gas).

* Atoms and Molecules: Gases are made up of atoms or molecules, which have mass. Even though gas molecules are spread far apart, the total mass of all the molecules in a sample of gas is a constant value.

* Measuring Gas Mass: You can measure the mass of a gas using a balance, a scale, or other methods. You can also calculate the mass of a gas using its density, volume, and molecular weight.

Key Point: While a gas may not have a definite volume or shape (it takes the shape of its container), it still has a definite mass.