Non-Newtonian fluids are substances that do not follow Newton's law of viscosity, meaning their flow behavior changes under stress or over time. Unlike Newtonian fluids, such as water or oil, which have a constant viscosity, non-Newtonian fluids can become thicker or thinner depending on the force applied. Common examples include cornstarch mixed with water (often called oobleck) and paint. These fluids can be categorized into different types based on their behavior. For instance, shear-thinning fluids become less viscous when stirred, while shear-thickening fluids become more viscous under stress. Understanding non-Newtonian fluids is important in various fields, including materials science and engineering.