Ball lightning - a slow-moving ball of light that is occasionally seen at ground level during thunderstorms - has puzzled scientists for centuries. There have also been reports of ball lightning in aircraft, but the origins of this phenomenon have remained a mystery. Now John Gilman of the University of California at Los Angeles has suggested that one of the properties of ball lightning - the cohesion that keeps the ball together for periods of tens of seconds - can be explained in terms of Rydberg atoms (J Gilman 2003 Appl. Phys. Lett. 83 2283). However, other researchers in the field disagree.