Fluid facts for kids

A fluid is a substance that can easily move and change its shape. Think of water, which is a liquid, or air, which is a gas. Even plasmas, like the stuff inside a neon sign, are fluids!

Fluids can "flow." The word "fluid" and "flow" actually come from the same old Latin word, fluere, which means "to flow." The study of how fluids move and behave is called fluid mechanics.

One big difference between fluids and solids is their shape. Solids, like a rock, keep their own shape. But fluids usually take on the shape of whatever container they are in. If you pour water into a glass, it fills the glass and takes its shape.

Some things are a bit tricky and don't fit perfectly into being just a fluid or just a solid. For example, Silly Putty can stretch and bounce like a solid, but it can also slowly flow like a very thick liquid over time. Powders, like flour or sand, are made of many tiny solid pieces. They can appear to flow, but each tiny piece is still a solid.

Most fluids have a special temperature called a freezing point. When a fluid gets cold enough to reach this temperature, it changes from a fluid into a solid. For example, water freezes into ice at 0 degrees Celsius (32 degrees Fahrenheit).

What are the Types of Fluids?

Fluids are usually divided into three main types: liquids, gases, and plasmas.

Liquids

A liquid is a fluid that has a definite volume but no definite shape. This means that a certain amount of liquid will always take up the same amount of space, but it will change its shape to fit its container. Water, juice, and oil are all common liquids. Liquids can be poured and they often have a surface.

Gases

A gas is a fluid that has no definite volume and no definite shape. Gases will expand to fill any container they are in. This is why air, which is a mixture of gases, fills up an entire room. You can't see most gases, but you can feel them, like when the wind blows. Oxygen and nitrogen are examples of gases.

Plasmas

Plasma is often called the "fourth state of matter" after solids, liquids, and gases. It's a very hot, electrically charged gas. Plasmas are found in places like lightning, neon signs, and even the stars! They behave like fluids because their particles can move around freely.

How Fluids are Used

Fluids are incredibly important in our daily lives and in many technologies.

In Nature

• Water is a fluid that is essential for all life on Earth. It flows in rivers, fills oceans, and circulates through our bodies.
• Air is a fluid that we breathe. It moves as wind, creating weather patterns and helping plants spread their seeds.
• Blood is a fluid that flows through our bodies, carrying oxygen and nutrients.

In Technology

• Hydraulics use liquids (like oil) to transfer force. This is how brakes in cars work, or how heavy machinery lifts things.
• Aerodynamics is the study of how air flows around objects. This is important for designing airplanes and cars to move efficiently.
• Pneumatics use gases (like air) to power tools and machines, such as air compressors or jackhammers.