A fluid is any body that has the property of flowing, and lacks rigidity and elasticity, and consequently yields immediately to any force tending to alter its shape and thus adopting the shape of the container that contains it. Fluids can be liquids or gases depending on the different intensity of the cohesion forces between their molecules.

In liquids, intermolecular forces allow particles to move freely, although they maintain latent bonds that make substances in this state have a constant or fixed volume. When a liquid is poured into a container, the liquid will occupy the volume equal to or partial to the volume of the container regardless of the shape of the container.

Liquids are incompressible because their volume does not decrease when very large forces are exerted on them. Another of their properties is that they exert pressure on the bodies submerged in them or on the walls of the container that contains them. This pressure is called hydrostatic pressure.

Gases, on the other hand, consist of well-separated moving particles that collide with each other and try to disperse, so that gases have no definite shape or volume. And so they take the shape of the container that contains them and tend to occupy the largest possible volume (they are very expandable).

Gases are compressible; that is, their volume decreases when forces are applied to them. For example, when force is exerted on the plunger of a syringe.

Fluid mechanics is the part of Physics that studies fluids both at rest and in motion, as well as engineering applications and mechanisms that use fluids. Mechanics is divided into fluid statics or hydrostatics, which deals with fluids at rest or in equilibrium; and in fluid dynamics or hydrodynamics, which deals with fluids in motion.

On the other hand, in terms of language, everything that comes easily and is well structured is said to be fluid; that is to say, to loose, current, easy and continuous language, without interruptions. For example: Maria has a very fluent German in her qualities.