Non-Newtonian fluids are substances that do not follow Newton's law of viscosity, which states that the viscosity of a fluid remains constant regardless of the applied stress. Instead, the viscosity of non-Newtonian fluids changes when force is applied. For example, when you stir cornstarch mixed with water, it behaves like a solid under quick movements but flows like a liquid when moved slowly. These fluids can be categorized into different types based on their behavior. Shear-thinning fluids, like ketchup, become less viscous when stirred, while shear-thickening fluids, like oobleck, become more viscous under stress. Understanding non-Newtonian fluids has applications in various fields, including food science and materials engineering.