You don't have to cite some things because they're common knowledge and are not considered the work of any particular person.

Examples of common knowledge are:

• There are four seasons in the year.
• There 365 days in a year.
• The U.S. entered World War II after the bombing of Pearl Harbor.
• The state bird of Georgia is the brown thrasher.

Source: http://www.usg.edu/galileo/skills/unit08/credit08_04.phtml

Common knowledge is information that the majority of people either know or can find in a number of sources. Common knowledge is factual information that is beyond dispute. Sure, you might not remember (or ever have known) what Georgia's state bird is, but you can easily look it up in an almanac, encyclopedia, the state's Web site, or other resource.

If you're not sure whether something is common knowledge or not, go ahead and provide a reference for it.

Source: http://www.usg.edu/galileo/skills/unit08/credit08_04.phtml