Make your own non-Newtonian slime.

Instructables has step-by-step instructions for making your own Rheopectic slime.

The slime we made is a Non Newtonian fluid and is rheopectic which means it shows an increase on its apparent viscosity with time under a constantly applied stress.

In other words, the more you play with it, it will become more viscous (even you can make a ball with it and it bounces) and we you stop (the shearing force is gone) it starts flowing (see the video, first you will see it flow when no shear force is present (you can also leave it in a table an see how it flows), then I applied a shear force on it and made it a ball that bounced.

This recipe is not safe for children under 5, who might eat it. Link