In science, the use of the word fluid applies to both liquids and gases. A fluid is a substance that does not have a fixed shape, and one whose molecular constituents move freely past one another. Thus, fluids take on the shape of their containers. The distinguishing feature between a liquid and a gas is that a liquid is a fluid that has a relatively fixed volume, and a gas is a fluid that does not have a fixed volume: gases can be compressed and can expand to fill a container entirely.
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One exception to this scientific use of fluid is bodily fluids. Here the word fluid applies to liquids only.
