Designing and writing the instructions that are part of the user interface design is both an art and science, involving copywriting and design skills as well as an understanding of how people use mental models. Mental models are a generalized idea of how things work. They are an efficient and speedy cognitive mechanism. People apply their mental models to new situations so they don’t need to relearn everything from scratch. This means people will apply their stereotype or mental model of similar websites to how your website works.

See original: The Small Print: Writing User Interface Instructions