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The substance is a mix of water and cornstarch. The mixture is two parts cornstarch to one part water. It demonstrates the properties of non-Newtonian fluids. Non-Newtonian fluids are solid if you apply a sudden force. and liquid if you apply a steady, slow force.

How can something be a solid and a liquid?

In the mix, there are tiny cornstarch particles evenly distributed.

When a steady, slow force is applied... the particles have time to move. So the object slides through, as it would in a liquid.

When a sudden force is applied... the particles don't have time to move out of the way. So the object stops, as it would on a solid surface.

What makes non-Newtonian fluids unique is viscosity.

Viscosity is the rate at which fluid flows. Common fluids, like water, have a consistent viscosity.

So they flow the same regardless of what force you apply.

In non-Newtonian fluids, viscosity can change.

Apply a sudden force, and the viscosity rapidly increases, forming a semisolid surface.

What can you do on a non-Newtonian fluid pool?

Jump. Bike. Flip.

Whatever you do, just keep applying high force.
Or you'll sink right in.