When the liquid moves, the internal friction between the liquid layers prevents them from moving relative to each other. This property is called viscosity.
The so-called liquid layer is artificial, that is, any liquid layer taken along the direction of movement in the fluid will have internal friction on the contact surface.
Viscosity can only be manifested when the fluid moves.
The internal friction is caused by the gravitational force between the molecules inside the liquid, that is, the gravitational force between the molecules on the interface of the taken liquid layer. The exchange of particles between liquid layers, that is, when the liquid flows, the molecules inside it have molecular movements different from the macroscopic flow direction. The molecules in the slow layer near the edge and the molecules in the fast layer near the center will exchange, resulting in the so-called momentum exchange, which reduces the speed of the fast layer on a macroscopic scale, acts as a resistance, and accelerates the slow layer, acting as a drag, which is the phenomenon of internal friction.
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