A fact that is considered common knowledge does not have to be cited. Facts are not original ideas.
Examples:
- Abraham Lincoln died on April 15, 1865.
- Planes crashed into the World Trade Center on September 11, 2001.
- The Pope lives in Vatican City.
How do I know if something is common knowledge?
Generally, something is considered general knowledge if it is known amongst a wide variety of people and can be found in a general reference resource, such as an encyclopedia, almanac or dictionary. If you aren't sure, it is better to play it safe and provide a citation.