Add corn starch to water and stir slowly. Keep adding until the suspension is near its maximum concentration – what happens next becomes rather interesting. It’d exhibit what is known in physics as “Non-Newtonian fluid” properties in which the viscosity changes with the applied strain rate.
In layman’s term, it means that if you apply abrupt force on it, the suspension behaves like a solid. For example, if you quickly poke it with a stick, the stick would bounce back. But if you slowly insert the stick into the fluid, the suspension would still behave like a liquid, and the stick would submerge.
This makes for interesting effects, like in the video above: it is entirely possible (or even, somewhat easy) to run on top of a corn starch pool. The first application that came to my mind is actually… freshman orientation games. But of course, the cleverer and more serious people are researching on this effect in applications like bullet proof vests – the armor would be soft and flexible in normal situations, but when impacted at high velocity by a bullet, that part of the vest would behave like a solid, repelling the bullet.