Wikipedia:You don't need to cite that the sky is blue

Verifiability is an important and core policy of Wikipedia. Article content should be backed up by reliable sources wherever needed to show that the presentation of material on Wikipedia is consistent with the views that are presented in scholarly discourse or the world at large. Such sources help to improve the encyclopedia.

Which of these things needs a citation?

However, many editors misunderstand the citation policy, seeing it as a tool to enforce, reinforce, or cast doubt upon a particular point of view in a content dispute, rather than as a means to verify Wikipedia's information. This can lead to several mild forms of disruptive editing which are better avoided. Ideally, common sense would always be applied but Wiki-history shows this is unrealistic. Therefore, this essay gives some practical advice.

Not citing common knowledge and not providing bibliographic entries for very famous works is also consistent with major academic style guides, such as The MLA Style Manual and the APA style guide.