Ocean waves off the coast of Mexico
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copyright 2002 University Corporation for Atmospheric Research

# What is a Fluid?

The short answer is that a fluid is a gas, a liquid or a plasma.

More technically, a fluid is something which can't support a shear stress. This sounds more complicated than it is. A shear stress is when you try to push an object in two different directions without squeezing it. For example, let's say that there's a pen lying on the table in front of you with the tip pointing away from you. Now let's say you push the tip of the pen to the left and the bottom of the pen to the right. You've just applied a shear stress. Now the pen is sideways (pointing to the left).

Now let's repeat this experiment with a fluid. We'll have to change the context a bit because any fluid on our table would need to be in a solid container to keep it from spilling or floating away. So instead we'll consider an object out in space - how about Jupiter? Let's imagine we're big enough to give Jupiter a shove. We can push the top (north pole) to the left and the bottom (south pole) to the right just like the pen. Would Jupiter turn sideways? The answer is no, at least not in the same way as the pen. Your hands would mostly go right through. You could move the molecules around but not without mixing them up. That's what we mean when we say a fluid can't support a shear stress. Jupiter is made of gas and a gas is a fluid.

Much of the universe is made of fluid, including planetary atmospheres and oceans, stars, and interstellar clouds. Even rock and metal can be fluid if they're hot enough to be molten; 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.

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