Viscosity

Viscosity is the internal friction of a flowing fluid. In a Newtonian fluid, the shear stress between two layers of fluid is proportional to the velocity gradient across them: τ = η·du/dy. The constant η is the dynamic viscosity, with SI units of Pa·s.

Water at 20 °C has a viscosity of about 1 mPa·s; air is about 18 μPa·s; honey is around 10 Pa·s — ten thousand times more viscous than water. Viscosity depends strongly on temperature: liquid viscosity decreases as you heat it (molecules slip past each other more easily), while gas viscosity increases (faster thermal motion transfers more momentum between layers).