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 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.

There are two types of viscosity: dynamic viscosity, measured in pascal seconds, and kinematic viscosity, measured in metres squared per second .^[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 root viscum, meaning sticky.^[4]

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