A mixture of cornstarch or potato flour and water in a ratio between 2:1 and 3:2.

"Though initially it acts like a viscous liquid, applying pressure quickly will make it appear to be a solid for a short time. The oobleck also behaves unlike conventional solids or liquid when thrown in the air, molded, heated, or vibrated. Substances that become more viscous when agitated or compressed are a subset of non-Newtonian fluids called dilatants. Oobleck contains no preservatives and is therefore perishable. "

http://en.wikipedia.org/wiki/Oobleck

"A non-Newtonian fluid is a fluid in which the viscosity changes with the applied shear stress. As a result, non-Newtonian fluids may not have a well-defined viscosity. Many polymer solutions and molten polymers are non-Newtonian fluids.

http://en.wikipedia.org/wiki/Non-Newtonian_fluid

See also the video: A pool filled with non-newtonian fluid