Oobleck.

Here's a recipe: http://www.livescience.com/21536-oobleck-recipe.html

Here's the cool part, "Oobleck is a non-Newtonian fluid; it has properties of both liquids and solids. You can slowly dip your hand into it like a liquid, but if you squeeze the oobleck or punch it, it will feel solid."

It is awesome.