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Illustration of viscosity; the violet fluid at the bottom has a higher viscosity than the white one at the top

Viscosity is a physical property of fluids. It shows resistance to flow.[1] In a simple example, water has a low viscosity, as it is 'thin'. Syrup or tar, on the other hand has a high viscosity, as it is '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, whereas water would be a lot quicker.

There are two types of viscosity: dynamic viscosity, measured in pascal seconds, and kinematic viscosity, measured in metres per second squared.[2]

Viscosity is used as a way to predict when volcanoes erupt. When the lava comes out very thickly (viscous), there is more chance that it will erupt violently. This is because the lava has a hard time getting out and may burst out when it can. If the lava is thin (low viscosity), then it just flows out like water.[3]

The word viscous comes from the Latin viscum, meaning sticky.[4]