Newtonian Fluids are fluids which have a different reaction to both stress and temperature. Sir Isaac Newton came up with Newtonian Fluids, and what he found is that The viscosity of Newtonian Fluids is affected by temperature of liquid; increasing temperature makes less viscous because the heat causes particles to move faster and there will be less attraction. Water, air milk, glycerol, alcohol, and thin motor oil are great examples of Newtonian fluids that have different density properties, which allows a different shear stress. This means that these fluids can become more viscous by cooling them. But, this is different when using gases. When using gases, there are no Non-Newtonian gases. This is because since gases all have no density, their viscosity cannot be changed using stress. With gases, everything is opposite. When increasing temperature, the gases become more viscous. This is because since you are making the particles move faster, they are very prone to collide with each other, causing "obstacles" that increase the viscosity of the gas in use. That is how the viscosity of Newtonian Fluids can be changed. When using Non-Newtonian Fluids, you can change the viscosity of the fluid by applying stress on it. This is because the sudden force that is applied onto the fluid could cause it to thicken, and become more viscous. An example of this is with a ketchup bottle. When you're ketchup gets stuck at the bottom and you hit the bottle, the ketchup suddenly falls to the bottom. This is because the stress that you apply causes the viscosity to decrease. Newtonian Fluids and Non-Newtonian fluids are important because fluids are sometimes used to build houses. An example of when a Newtonian Fluid must be used is if I was building a house, and used clay as the base, if I were to apply to much stress onto it, it would become a runny mixture. Our friends over at the mythbusters did a very interesting experiment which you can check out here.