When does spilling water not make something wet? This question may sound like the beginning of a riddle, but scientists at the College of France in Paris have actually found a way to move a liquid across a surface while keeping it dry. Their results appear in today's issue of the journal Nature.

Pascale Aussillous and David Quere coated small amounts of fluid with a hydrophobic, or "water fearing," powder to make "liquid marbles" that can roll over surfaces without leaving a trace. A thin layer of the powder settles between the liquid and the surrounding air and allows the coated water to retain a spherical shape. The drops can then remain on a variety of surfaces¿including a pool of water¿without "spilling."

The scientists say their liquid marbles behave more like soft solids than liquids. When regular water droplets interact with a solid surface, such as a pane of glass, they form a lens shape and tend to move by sliding¿in which case some liquid gets left behind, wetting the surface. But when the liquid marbles come in contact with a solid surface, their spherical shape results in less interaction and, instead of sliding, they roll.

Moving the liquid marbles is easy, the researchers say, and can be done using small gravitational, electrical or magnetic fields. They hope the liquid marbles will find uses in technological applications that need small amounts of liquid moved quickly across a solid surface. But before such applications can emerge, L. Mahadevan of the University of Cambridge writes in an accompanying article, researchers need to investigate how sturdy the drops are and how they will behave with age.