Interactivity describes a relationship between two entities that mutually affect one another. It can be viewed from three perspectives: as a formal property of media technology, as a communication process between two actors, or as an information process within an individual user. The degree of interactivity is determined by criteria on a continuum, indicating which properties make a medium more or less interactive, such as reciprocity, responsiveness, speed, and non-verbal communication. Interactivity can occur between humans, humans and documents, or humans and systems. It can also be open and allow for productive new interactions, or closed and restricted to existing content.