τ=μ(du)/(dy)；

Therefore, the velocity gradient in the vertical flow direction determines the existence of viscous force, and there is no velocity gradient at rest, so there is no viscous force. According to Newton's law of interaction, the corresponding shear stress does not exist.

Force is the embodiment of the essence of flow.

Viscosity is the nature of the fluid itself, the embodiment of physical properties, and it exists forever.