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Illustration of viscosity; the violet fluid at the bottom has a higher viscosity than the clear one above

Viscosity is a physical property of fluids. It shows resistance to flow. In a simple example, water has a low viscosity, as it is "thin". Syrup and tar, on the other hand, have a high viscosity, as they are "thick". A way to test for viscosity is the speed at which the substance runs down a slope. Syrup would reach the bottom very slowly, and water would be much quicker.