Non-Newtonian fluids are those that behave differently than Newtonian fluids do with regard to viscosity. Typically, stress causes non-Newtonian fluids to alter their flow patterns or viscosity. The stress can cause the viscosity to increase or decrease, depending on the particular non-Newtonian fluid.
Tomato sauce is an example of a non-Newtonian fluid which exhibits decreased viscosity when stress is applied to it. For example, when trying to get tomato sauce out of a bottle, people often hit the bottle. This stress causes the tomato sauce to behave as more of a liquid. When the stress acting on the tomato sauce is removed, it begins behaving like a solid again.