Things that are considered to be common knowledge do no need citations. Citations are necessary when the information you are sharing is not something you knew prior to your research, or is someone else's original idea or product of their own research.
- QUOTE: Pulling a sentence(s) directly from a document and putting into your own words (always within quotation marks)
- PARAPHRASE: Restating an idea from the original source in your own words
- SUMMARIZE: Taking the main ideas from the original source and putting them into your own words
The sky is blue. --> This is common knowledge, so no citation needed.
Sunlight reaches Earth's atmosphere and is scattered in all directions by all the gases and particles in the air. Blue light is scattered more than the other colors because it travels as shorter, smaller waves. --> This is not common knowledge, but instead fact derived from original research and would require a citation.