The only source material you can legitimately use without quotation. Common knowledge is information that could be generally known to an educated reader. Examples of this might be facts such as Paris is the capital city of France or widely known historical or scientific facts, such as the molecular structure of water is H2O. However, ideas or interpretations would not normally be considered common knowledge. If you are in doubt as to whether something in your discipline is common knowledge or not it would be best to reference your source. When to Cite page provides more examples for common knowledge.