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What is a Fluid? - Windows to the Universe

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What is a Fluid?

A fluid is anything that would spill or float away if it weren't in a container (unless it's big enough to be held together by gravity like a star). If you can stir it up with a spoon or blow it through a straw, it's a fluid. Water is a fluid and so is air. In fact, all liquids and gases are fluids. In space and inside stars there's also another kind of fluid called a plasma.

The molecules in a solid are stuck together, but in a fluid they're free to move past each other. So if you had very small hands you could push one molecule of a fluid one way and another molecule the other way and off they would go in the direction you pushed them.

Much of the universe is made of fluid, including Earth's atmosphere and oceans, giant planets like Jupiter, stars like the Sun, and huge clouds of gas and dust in space. Even rock and metal can be fluid if they're hot enough; that's what happens deep inside the Earth.

Fluid dynamics (also called fluid mechanics) is the science of how fluid moves around. A fluid in motion is called a flow.

Last modified November 2, 2005 by Jennifer Bergman.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF