Common knowledge means facts which people know without having to look them up, such as the capital of Kansas is Topeka. You do not have to cite a source of information that is common knowledge.
Washburn University defines plagiarism as, "...the appropriation of another person's work, with or without that person's consent, and the unacknowledged incorporation of that work into one's own work offered for credit." Washburn Faculty Handbook, 10th Edition
