Insulating materials, which are those that resist the flow of electricity, accumulate electrical charges when rubbed or slid against another surface, or when indented or bent1–3. This phenomenon is known as contact electrification or triboelectric charging. Examples in everyday life can be both amusing and dramatic4: hair standing on end, a party balloon sticking to walls and, at larger scales, lightning and dust storms. However, triboelectric charging is hard to predict. A particularly puzzling example is the asymmetric charging of two objects made from the same material which, according to simple theory, should not occur. Writing in Nature, Grosjean et al.5 report that surface contamination from carbon compounds, which is present on nearly all objects that are exposed to air, can cause this same-material contact electrification.…