They filled a pool with a mix of cornstarch and water made on a concrete mixer truck. It becomes a non-newtonian fluid. When stress is applied to the liquid it exhibits properties of a solid.
Wikipedia: http://en.wikipedia.org/wiki/Non_Newtonian_fluid
Recorded at Barcelona, Spain.
TV Show: http://www.cuatro.com/microsites/el_hormiguero/cientifico.html
