The most common of a non-Newtonian fluid is a suspension of starch (e.g. cornflour) in water. The sudden 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. [Wikipedia] That is what you will see in the video below. It really looks great, so don’t miss it!
Here is how to make it for yourself. Mix equal amounts of starch and water, perhaps a little less water, then stir.