The row over who will provide the anti-graffiti technology for Berlin's Holocaust memorial highlights a widespread and ancient problem. Víctor Castaño and his colleagues at the Autonomous National University of Mexico, in Santiago de Querétaro, believe they have a solution - a paint to which the daubings will not stick. Deletum 5000's special ingredient is silica. It is loaded with particles of the stuff that are but a few nanometres across. These particles have had both oil-repellent and water-repellent molecules attached to their surfaces.