Viscosity is a measure of a fluid's resistance to flow. It describes the internal friction of a moving fluid. A fluid with large viscosity resists motion because its molecular makeup gives it a lot of internal friction. A fluid with low viscosity flows easily because its molecular makeup results in very little friction when it is in motion.
Consider how these liquids look when you pour them:
- Egg yolk
Newtonian Liquid: only temperature affects its viscosity
Non-newtonian liquid: other forces/factors affect its viscosity
Examples of Non-Newtonian Liquid:
Corn starch and water
When a force is applied to the polymer chains, they straighten out and entangled with each other, raising the viscosity
There is a product that is used in some safety equipment (knee pads, maybe helmets) that is like cornstarch and water and works to absorb the force and protect you called D3O. See https://www.acs.org/content/dam/acsorg/education/resources/highschool/chemmatters/issues/2016-2017/February%202017/chemmatters-feb2017-d3o.pdf for more details.
This next video demonstrating D3O is not something you can try yourself. Watch this only after asking permission from your parents when you are at home.