What’s a Non-Newtonian Fluid? Luckily Wikipedia has that very answer.
A non-Newtonian fluid is a fluid whose flow properties cannot be described by a single constant viscosity. An inexpensive, non-toxic example of a non-Newtonian fluid is a solution of corn starch (corn flour) and water, sometimes called oobleck. The application of force - for example by stabbing the surface with a finger, or rapidly inverting the container holding it - leads to the fluid behaving like a solid rather than a liquid. This is the “shear thickening” property of this non-Newtonian fluid. More gentle treatment, such as slowly inserting a spoon, will leave it in its liquid state. Trying to jerk the spoon back out again, however, will trigger the return of the temporary solid state. A person moving quickly and applying sufficient force with their feet can literally walk across such a liquid.
