A mixture of corn starch and water. It can behave as a liquid or solid, depending on how much force you apply. Roll it around in your hands, and it forms a ball. Let it sit, and it melts into a puddle. Great fun.

To make it, start out with some corn starch and add water little by little, stirring constantly. Stop when the water and corn starch are completely mixed, but before it's too easy to stir. Add green food coloring for the complete experience.