When you add the water to the polymers in the slime powder, it helps link the polymer molecules together. When enough of the molecules get hooked together, the solution changes to slime!

Viscoelastic Slime is classified as a 'Non-Newtonian' fluid. When a force is applied to them - such as hitting them or attempting to stir them vigorously - they harden and behave like solids. Remove the force, and they begin to flow like liquids. Viscoelasticity is a property of materials that show both viscous (resistance to deformation under stress) and elastic (resistance to distorting stresses and the ability to return to the original shape when stress is removed) properties.