Simply put, a fluid is any substance that can flow. This includes both liquids, like water, and gases, like the air around us. The defining feature of a fluid is that it cannot hold its own shape; it will always take the shape of the container it's in.

Think about a solid block of wood. You can place it on a table, and it will sit there, maintaining its shape. Now, imagine pouring a glass of water onto the same table. The water doesn't hold its shape; it spreads out and flows. A gas is even more dramatic. If you release a gas into a room, it won't just sit in one spot—it will expand to fill the entire space.

The difference between solids, liquids, and gases comes down to how their molecules are arranged and how they interact.

• Solids: Molecules are packed tightly in a fixed structure. They can vibrate, but they don't move from their positions. This is why a solid has a definite shape and volume.

• Liquids: Molecules are close together but are free to move past one another. This allows a liquid to flow and take the shape of its container, but it still has a definite volume.

• Gases: Molecules are far apart and move randomly and rapidly. They are not bound to each other, which is why a gas will expand to fill any container, having neither a fixed shape nor a fixed volume.

Because both liquids and gases have molecules that can move around freely, we group them together under the single category of fluids. Fluid mechanics is the study of how these substances behave, both at rest and in motion.

The technical reason fluids flow has to do with something called shear stress. This might sound complicated, but the idea is simple.

Imagine placing a book on a table. If you put your hand flat on the cover and push sideways, you're applying a shear stress. The book might slide, but it resists your push and moves as a single object. A solid object resists this kind of sideways force.

Now, try to do the same with water in a bucket. If you slide your hand across the surface, the water doesn't resist in the same way. It flows. The layer of water under your hand moves, and it drags the layer below it, which drags the layer below that. The water continuously deforms as long as you apply that sideways force.

Even though we know fluids are made of countless individual molecules, it's often useful to pretend they're not. In fluid mechanics, we usually treat a fluid as a continuum. This means we look at it as a smooth, continuous substance rather than a collection of separate particles.

Think of a digital photograph. If you zoom in far enough, you'll see it's made of tiny squares called pixels. But from a normal distance, you see a smooth, continuous image. Treating a fluid as a continuum is similar. It allows us to describe its properties, like density or pressure, at any point within the fluid, without having to track every single molecule. This simplification makes studying the motion of fluids much more manageable.