Viscosity is the property of gases and liquids to resist flow or relative motion of adjacent layers. Viscosity is a form of internal friction of a fluid. A quantitative measure of internal friction is the coefficient of viscosity of a liquid. Viscosity depends on the structure of the substance and the temperature (it decreases significantly with increasing temperature). Viscosity is the friction that occurs when liquid molecules move their layers during flow (deformation) under the influence of an appropriate stress. It occurs only when the fluid is in motion. This happens when the liquid layers move at different speeds. Since printing inks contain suspended pigment particles, which are like plastic, they have a great influence on the measurement of viscosity - the relative elasticity of the ink. This means that a certain force must be applied for the metal cylinder of the viscometer to overcome the relative elasticity of the ink before the cylinder is activated. The force required to overcome the relative elasticity (stiffness) of the coating is called the yield strength. The yield strength of ink depends on the amount of pigment, the particle size and shape of the pigment, and the degree to which it is dispersed or flocculated in the binder. The viscosity of dense printing inks is determined by measuring the resistance of a metal bar through the ink in a unit of time. For rare printing inks, a simple hand-held viscometer is used, a different discharge cup with a nozzle at the bottom of the measuring cup. It measures the flow of the ink volume in a unit of time through an orifice of a certain diameter. The viscosity of printing inks can also be determined with a rotational viscometer by measuring the resistance that the liquid offers to the rotation of the metal axis/spindle.