You can use common knowledge without referring to a source. This concerns knowledge that many people are familiar with, or which can easily be found in a general encyclopedia.
For example, you don’t need to cite the following:
- Generally accepted dates for military, political, literary and other historic events:
In 1865, following the assassination of Abraham Lincoln, Andrew Johnson was elected as the 17th President of the United States.
- Date of birth and death for well-known people:
William Shakespeare was born in 1564 and died in 1616.
- Facts you can find in many different standard works:
DNA and proteins are the main constituents of chromosomes.
- General observations that anyone can make:
Little babies may cry for many different reasons.
- Common traditions and folklore:
Little Red Riding Hood and her grandmother were eaten by the wolf and saved by a hunter.
In each subject field there is also a mass of knowledge which is seen as common knowledge among those people who work in this field. It may include facts, methods and theories. What is seen as common knowledge differs between courses depending both on which subject you are studying and what level you are at.
For example you probably need to cite the source for information about the different signal substances of the brain if you are taking a course in pedagogy, while this might be considered common knowledge in an advanced course in neurobiology. However, you must be certain that the knowledge is well-known in the subject field that your readers or your audience are familiar with it.
Even if you are using common knowledge you still need to use your own words and not copy the wording of other sources.