Answer:
You'll need 1 tablespoon of cornstarch, 1 tablespoon of water, and a couple drops of cooking oil. Just mix everything up and microwave it for about 20-30 seconds on high.
Then take it out, let it cool for about a minutes, and kneed the plastic with your hands. You'll be able to mold it into a shape, and after 20 hours or so, it should be hard.
It will decompose and disappear if you bury it in your yard ;)