[1] Common knowledge can be about a broad range of subjects, such as science, literature, history, or entertainment.
[2] Assigning something the label of common knowledge requires certain considerations about the involved community, group, society and/or individuals, the time period, and the location.
[3] The variation can come from the time period, culture, population, class, age, demographic, and other circumstances.
Many techniques have been developed in response to the question of distinguishing truth from fact in matters that have become "common knowledge".
In legal settings, rules of evidence generally exclude hearsay, which may draw on "facts" someone believes to be "common knowledge".