|Like walking on water: A person plays on something|
called non-Newtonian fluid.
It's called "non-Newtonian fluid." (The things you discover when you're bored and wandering aimlessly around the Internet!)
Apparently, non-Newtonian fluid is a fluid that changes its viscosity whe you apply force to it, like hitting, shaking or jumping on it. This sudden application of stress makes the fluid get thicker and act like a solid.
This knowledge is not new. Scientists and others have pretty much been familiar with this phenomenon forever, practically.
What is new, at least new to me, is that people have now been using non-Newtonian fluid for fun and games. People fill pools with stuff, it mixes with water and becomes non-Newtonian fluid.
I wonder if it's cheap to buy this stuff. Apparently you can mix water and cornstarch to make a non-Newtonian fluid but you need a lot of it to get enough to have any fun.
It would certainly make a backyard swimming pool more interesting in the summer.
It looks really cool and fun, quite frankly, as the video below indicates: