What is a fluid?
Why water and air can flow, and why that matters for every pump in the world.
⏱ 4 min
- Everyday comparisonStep 1 of 5
Imagine a bag of marbles
Pour a bag of marbles onto a table. They roll, spread out and slip past each other. Now imagine a brick: it keeps its shape no matter what.
Water behaves like the marbles. Its tiny pieces — called — can slide past each other. That is why water can flow.
- WatchStep 2 of 5
Solid, liquid, gas
Here are the three ways stuff can exist. Switch between them and watch the little dots (the molecules).
In a solid, the dots are locked in place and only jiggle. In a liquid, they stay close but slide around. In a gas, they fly everywhere and fill the whole box.
Challenge: Switch to “Solid”, then “Gas”. Which ones fill the bottom of the box, and which one fills all of it?
- Try itStep 3 of 5
Tilt the glass
A liquid always takes the shape of whatever holds it. Tilt the glass and watch the water settle into the new low corner.
Anything that can flow like this is called a . Both liquids and gases are fluids.
Challenge: Tilt the glass all the way left, then all the way right. Does the water ever keep its old shape?
- ExplainedStep 4 of 5
Thick and thin fluids
Some fluids flow easily, some are lazy. Water pours fast; honey pours slowly. Engineers call this “thickness” .
Here is one more secret: you can squeeze air into a smaller space (think of a bicycle pump), but you cannot squeeze water. Water is . Remember that — it becomes important later.
- The impeller connectionStep 5 of 5
Why this matters for impellers
An is a machine that pushes fluids around: water in your washing machine, coolant in your car, air in your vacuum cleaner.
It only works because fluids can flow. You could never “pump” a brick!
📌 Remember this
A fluid is anything that flows and takes the shape of its container — liquids and gases. Impellers are machines that move fluids.