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Matter may be divided into four phases or states: solid, liquid, gas and plasma (ionized gas). The termfluid has been used as a general name for the last three states: liquid, gas, and plasma because they may be deformed without applying any force, provided that the change of shape occurs very slowly. Furthermore, when a large number of small solid particles flow in a fluid, if the velocity of the fluid is sufficiently high, the behavior of such solids is similar to ordinary fluid. We may consider these solid particles as pseudofluid. Under proper conditions, we may treat fluid flow problems for solid (pseudofluid), liquid, gas and/or plasma. In ordinary fluid mechanics, we treat the flow problems of a fluid which is in one state only, i.e., liquid, gas, or plasma, and the solids in this fluid medium are assumed to be rigid bodies which may be considered as given boundary conditions of the flow problems.