Viscosity: A fluid in motion, can be considered as composed of many layers. If a fluid moves in steady flow over a static horizontal plane, the layer touching the static plane has zero velocity, while the layer next to the layer in contact moves with a least velocity. The velocity of the layer increases with the distance from the static plane (fixed surface). If any two layers of a fluid in motion are considered, the upper layer tends to accelerate the motion of the lower layer and lower layer tends to retard the motion of the upper layer. This property of a (liquid) fluid which opposes the relative motion between the different layers is called viscosity or internal friction. Generally, liquids of low density like alcohol, water, spirit etc. are less viscous whereas, the heavy liquids like coal tar, mercury, honey etc. are more viscous.