A documentary film is a non-fictional motion-picture intended to "document reality, primarily for the purposes of instruction, education, or maintaining a historical record" - compare documentary theatre. Bill Nichols has characterised the documentary in terms of "a filmmaking practice, a cinematic tradition, and mode of audience reception [that remains] a practice without clear boundaries". Documentary films, originally called "actuality films", lasted one minute, or less. Over time, documentaries have evolved to become longer in length, and to include more categories; some examples being: educational, observational, and docufiction. Documentaries are meant to be informative works, and are often used within schools, as a resource to teach various principles.