Some examples of highly viscous liquids are oils, honey, glycerin, tar and sulfuric acid. Viscosity is the ability of substances, especially fluids, to resist flow. It can also be referred to as the measure of the ability of a liquid to resist being deformed by extensional stress.
The general understanding of viscosity is that it refers to the thickness or thinness of a fluid. Viscosity is determined by the friction between particles of a fluid. The shape and size of these particles will influence the level of viscosity in the fluid. There are other fluids that do not have any resistance or viscosity, and they are called ideal fluids.
Viscosity can be measured by a rheometer. In the absence of this equipment, engineers put a ball in a container filled with the particular liquid and let the ball travel down the fluid and measure the time the ball takes to reach the bottom of the container. The ball is thrown in the liquid at different velocities, and a proper Newton liquid will show the same viscosity even with different velocities. If a liquid displays different viscosities with changing velocities, then they are known as shear thickening or shear thinning. Understanding viscosity of fluids helps in mining and selection of fluids to be used in equipment like car brakes.