When we handle an object it is the surface that we touch. The nature of that surface determines some of the vital properties of the object: its appearance, its friction properties and whether we can stick things to it or print on it. But what exactly is a surface – how deep does one go before the surface becomes the bulk? This is an old question for metals, semiconductors and other non-polymeric solids, and the answer is typically a few atomic layers, perhaps a few tenths of a nanometre.