Together, liquids and gases are known as fluids.
At room temperature water is a fluid and liquid, air is a fluid and a gas.
Solids keep definite shape by resisting forces that try to change their shape or deform them.
Fluids can flow to fill a container of any shape.
This is because the molecules which make up the fluid are able to move freely past each other rather than being held rigidly to each other by the forces between them as they are in a solid.
Because they can change shape or deform to fill a different shaped space, fluids are useful in driving machinery.
In a vehicle's braking system for example the braking force is transmitted from the driver's brake pedal to the brake pads through the pressure in a liquid known as the brake fluid.
Fluids flow easily. Any force aur pressure will change the shape of a fluid.
It will return to its former shape when the pressure is removed.
Air is a fluid. When air flows round the object the air flow changes.
The flow around an aircraft's wing is streamlined until the aircraft flies at a steep angle.
Then the air flow becomes turbulent thus increasing the drag.
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