Whether you call it "magic mud" or (my personal favorite) "oobleck", something uncanny happens when you combine 1 part water to 2 parts cornstarch. This Non-Newtonian fluid can flow, pour, and acts just like a liquid until a shearing force is acted upon it. As the starch molecules compact faster than the water around it, the substance becomes as hard as stone. It's that liquid/solid dichotomy that has fascinated children and science geeks for generations.
Here's just SOME of the weird things you can do with it!