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A fluid is a phase of matter and include liquids, gases, plasmas and, to some extent, plastic solids.

Fluids share the properties of not resisting deformation and the ability to flow (perhaps otherwise described as their ability to take on the shape of their containers). These properties are typically a function of their inability to support a shear stress in static equilibrium. While in a solid, stress is a function of strain, in a fluid stress is a function of rate of strain. A consequence of this behaviour is Pascal's law which entails the important role of pressure in characterising a fluid's state.

Fluids can be characterised as:

- depending on the way stress depends on strain and its derivatives. The behaviour of fluids is described by a set of partial differential equations, the Navier-Stokes equations.

Fluids are also divided into liquids and gases. Liquids form a free surface (that is, a surface not created by their container) while gases do not.

The distinction between solids and fluids is not so obvious. The distinction is made by evaluating the viscosity of the matter: for example silly Putty can be considered either a solid or a fluid, depending on the time period over which it is observed.

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