Researchers have come up with a reason why sand grains can build up electrical charges as they collide with one another -- sometimes to the point of creating lightning during dust storms and volcanic eruptions. A paper in the upcoming issue of the journal Nature Physics says that particles may transfer their charge vertically during a smashup, so that positive charges go down, while negative charges go up. The findings could have implications for a wide variety of problems, from the electrical charge emitted by a helicopter's takeoff to the adhesion of dust to solar panels on the Mars rover.