Day 128: Common knowledge does not need a reference.

Although we recommend providing references for new or controversial information, we do not recommend providing references for common or widely accepted knowledge. We also do not recommend providing a reference for commonly accepted definitions or descriptions.

For example, you do not need to provide a reference for this information: “Clouds are formed from water molecules.” This is common knowledge.

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You also would not need a reference for this information: “Books are composed of bound papers, which often contain words or images.”

Basically, if your readers already know, understand, and believe the information, you will not need a reference.