This is a regular activity at our house, everyone loves it! I don't have specifics, just mix some cornstarch and water together until the consistency is right. Add water a tablespoon at a time so you don't end up with to much liquid. Of course, if it gets a bit watery, just add more cornstarch!
This stuff is especially cool because it defies most rules of science! When force is applied, the mixture becomes a solid, but when force is removed, it instantly turns into a liquid.
You can demonstrate this by gently pushing on the surface of the mixture, you should feel resistance and be unable to push your finger tip through the liquid. Then hold your finger in one place for several seconds and the solid will appear to melt and your finger will sink right in.
This stuff can create a giant mess, but it wipes up extremely easily.
Occasionally we add food coloring to spice things up a bit. I haven't tried saving it yet, so I don't know how well it would keep for future play.
DO NOT DUMP THIS MIXTURE DOWN ANY DRAIN!!!
The liquid will run off and the cornstarch will turn into rock. We either dump it outside, or let the mixture sit over night. The liquid will evaporate and you can scrape the remaining solid right off with a spatula, and into the garbage can.