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A fluid is a substance that can easily move and change shape such as water (liquids), or air (gases), or plasmas. They can 'flow'- flow and fluids both come from the same English word roots (Latin: fluere "to flow") and are pronounced similarly.
Fluids usually take on the shape of the objects that they touch, in contrast to solids which maintain their own shape.
If you cool a fluid down enough it eventually becomes solid.