The ancient Greeks first recognized static electricity, but the mechanisms behind it remain somewhat mysterious. In particular, it's unclear how two pieces of the same material can build a charge between them simply by touching. Yet we regularly see examples of this when volcanic ash creates enough charge to discharge lightning. (Image credit: volcano - M. Szeglat, experiment - G. Grosjean and S. Waitukaitis; research credit: G. Grosjean and S. Waitukaitis; via APS Physics)