  We report the observation of upstream transport of floating particles when
clear water is poured on the surface of a flat water surface on which mate or
chalk particles are sprinkled. As a result, particles originally located only
at the surface of the lower container can contaminate the upper water source by
"riding" on vorticial water currents. We speculate that Marangoni forces in
combination with geometry-induced vortices may explain the observed phenomenon.
