Non-Newtonian fluids are substances that do not follow Newton's law of viscosity, meaning their flow behavior changes under stress or force. Unlike water, which flows consistently regardless of the applied pressure, non-Newtonian fluids can become thicker or thinner depending on how they are manipulated. Common examples include cornstarch mixed with water, which behaves like a solid when struck but flows like a liquid when handled gently. These fluids can be categorized into different types based on their response to stress. For instance, shear-thinning fluids, like paint, become less viscous when stirred, while shear-thickening fluids, like oobleck, become more viscous under pressure. Understanding non-Newtonian fluids has applications in various fields, including food science and materials engineering.