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The characteristic that is not considered a fundamental property of a gas is the concept of "particles." While gases are composed of particles, this term refers more generally to the substances that make up matter, rather than a specific property of gases themselves.

In understanding gas properties, volume, mass, and pressure are critical:

- Volume refers to the space that the gas occupies. Gases can expand to fill any container, which means their volume is not fixed and depends on the container's size.

- Mass is a measure of the amount of matter in the gas. All gases, like any matter, have mass, and this can influence behaviors in physical and chemical processes.

- Pressure is the force that gas particles exert on the walls of their container. It is a fundamental property that results from collisions of gas particles with the surfaces surrounding them.

Therefore, while the gas consists of particles, simply identifying particles does not convey a specific property of gases, making it the correct answer to the question.