Writing is a cognitive activity involving neuropsychological and physical processes and the use of writing systems to structure and translate human thoughts into persistent representations of human language. A system of writing relies on many of the same semantic structures as the language it represents, such as lexicon and syntax, with the added dependency of a system of symbols representing that language's phonology and morphology. Nevertheless, written language may take on characteristics distinctive from any available in spoken language. The outcome of this activity, also called "writing", and sometimes a "text", is a series of physically inscribed, mechanically transferred, or digitally represented linguistic symbols. The interpreter or activator of a text is called a "reader". Writing systems do not themselves constitute languages (with the debatable exception of computer languages); they are a means of rendering language into a form that can be reconstructed by other humans separated by time and/or space. While not all languages use a writing system, those that do can complement and extend capacities of spoken language by creating durable forms of language that can be transmitted across space (e.g. written correspondence) and stored over time (e.g. libraries or other public records). Writing can also have knowledge-transforming effects, since it allows humans to externalize their thinking in forms that are easier to reflect on, elaborate, reconsider, and revise. Tools, materials, and motivations to write Any instance of writing involves a complex interaction among available tools, intentions, cultural customs, genres, tacit and explicit knowledge, and the constraints and limitations of the writing system(s) deployed. Inscriptions have been made with fingers, styluses, quills, ink brushes, pencils, pens, and many styles of lithography; surfaces used for these inscriptions include stone tablets, clay tablets, bamboo slats, papyrus, wax tablets, vellum, parchment, paper, copperplate, slate, porcelain and other enameled surfaces. The Incas used knotted cords known as quipu (or khipu) for keeping records. Writing tools and surfaces have been countlessly improvised throughout history (as the cases of graffiti, tattooing, and impromptu aides-memoire illustrate). The typewriter and subsequently various digital word processors have recently become widespread writing tools, and studies have compared the ways in which writers have framed the experience of writing with such tools as compared with the pen or pencil. Word processors include, often multi-document, text editors or note-taking apps, Web systems (search engines, Wikis, etc) and messaging software (chat apps, e-mail UIs, etc) or their underlying operating systems' code