You do not need to cite common knowledge. Something is common knowledge if it can be found in many different sources and is a well known fact.
Examples:
Go through the tutorial Plagiarism tutorial from Academic Writer! While the tutorial focuses on APA style citations, the tips on how to avoid & identify plagiarism can be applied to all citation styles.
"The act of copying, stealing, or representing the ideas or words [images or sounds] of another as one's own, without giving credit to the source." (SJC Student Handbook)
Intentional Plagiarism
"The direct act of knowingly presenting another's words, creativity, research findings, or ideas as one's own in either exact or paraphrased form. In short, plagiarism in its most serious form involves falsely taking credit for someone else's thoughts and efforts."
"Presenting other's words, key ideas, or research findings without appropriate and complete acknowledgement. Various types of miss-attribution might be a matter of error or oversight, but failing to set quotation marks around a directly quoted excerpt from another's work or omitting a correct citation, whatever the intent, constitutes plagiarism."
(APA Style of Documentation: A Pocket Guide, 2009, p. 4-5)
Four main ways of incorporating sources into your paper:
Represent the original source accurately, and cite it carefully!!!
Definitions excerpted from "APA Style of Documentation: a Pocket Guide" by M. Pringle & J. Gonzales (2010)
"PLAGIARISM AND OTHER FORMS OF ACADEMIC DISHONESTY"
The University expects students to observe academic integrity in all aspects of their academic life, including the conduct of their examinations, assignments, and research. All members of the University community share the responsibility for creating a climate of academic integrity, based on fairness to others and respect for oneself. Violations of academic integrity are treated very seriously. Plagiarism (the act of copying, stealing or representing the ideas or words of another as one’s own without giving credit to the source), cheating on examinations, and all forms of academic dishonesty are forbidden. Students found guilty of such behavior are subject to appropriate disciplinary actions, which may include a reduction in grade, a failure in the course, suspension, or expulsion.
Instructors at St. Joseph’s University. NY routinely use plagiarism detection devices such as Turnitin, Safe Assign, and Google to uncover acts of plagiarism.