A Non-Newtonian Fluid is a type of fluid whose viscosity changes under stress or strain. Unlike water or oil, which have a constant viscosity regardless of the force applied, non-Newtonian fluids can behave differently. For example, when you stir cornstarch mixed with water, it becomes thicker and resists flow, but when left undisturbed, it flows easily. These fluids can be categorized into various types, such as shear-thinning and shear-thickening. In shear-thinning fluids, like paint, the viscosity decreases with increased shear stress, making them easier to spread. In contrast, shear-thickening fluids, like oobleck, become more viscous when force is applied.