Fluid bodies do not sustain the shear forces of movement. In other terms, the study of fluid dynamics is the study of movement when every movement is allowed.
∂u/∂t + (u∙∇)u = (-1)/ρ ∇P + ν∇² u + f
The Navier-Stokes equation above is a fundamental model stating that the velocity field (fluid motion), is the result of the equilibrium between three actions:
(-1)/ρ ∇P is the gradient of pressure: the perceived difference between where the fluid is, and the array of possibilities. Fluid motion tends towards a place of lower pressure, if this place exists. is the fluid language: pressure waves propagate from one particle to the next. Compression omens an upstream obstacle, and expansion informs that a crisis has passed.
+ ν∇² u is the body's viscosity, its resistance to change. Viscous bodies move with effort as their elements try to keep cohesion. Non-viscous bodies are incoherent and overly-excited with any energy input.
+ f is an external force, like gravity, which acts evenly on every fluid body but with different results, depending on their nature.
Like every other body of water, I too change with shear. I too wish to move towards lower pressure, more comfortable situations. Adverse gradients can decelerate me, detach me from my path, triggering instabilities and turbulence. I react differently to external forces that apply evenly on my peers: it doesn't take much for us to be influenced into opposite paths. My viscosity does keep my internal cohesion, but stops me from necessary change.