From: Harris, Robert A. The Plagiarism Handbook. Los Angeles: Pyrczk Publishing, 2001.
If it's not your idea and it's not something most people know about, then you need to cite it.
Common knowledge: refers to commonly known information about current events, famous people, geographical facts, or familiar history; also, an easily observed or commonly reported fact or common saying.
Exception: if you use someone's words (for ex. from a commentary, interpretation, analysis, etc.) even if they contain information that is common knowledge, you must cite the source.
Rule to follow: "If in doubt, cite it."