Viscosity signifies a fluid's resistance to flow, stemming from internal friction between its layers, akin to friction encountered by solids moving on surfaces. It's measured in poise (Pl) or Pascal seconds (Pa s), with dimensions [ML^-1T^-1]. Notably, liquids become less viscous with temperature rise, while gases exhibit increased viscosity. For example, water's viscosity drops from 1.0 centiPoise at 20°C to 0.3 at 100°C, whereas air's viscosity slightly increases from 0.017 to 0.019 centiPoise from 0°C to 40°C. Intermolecular forces play a critical role in determining a fluid's viscosity.