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|This page in a nutshell: Just because something appears obvious to you, doesn't mean it's obvious to everyone. Build articles from reliable, expert sources, and cite those sources.|
It is sometimes felt that "obvious" statements, such as "the sky is blue", do not need citing. However, there are some reasons why you do need to cite the "obvious", such as that the sky is blue.
First of all, you do need citations in the "main" article, i.e., where the subject is the "obvious" statement or its major element. I.e., the statement "the sky is blue" must be footnoted in the article "Sky", especially in the section which discusses the color of the sky. Such references usually lead to more detailed knowledge.
Even the most obvious and simple assertion may need an explanation. The statement that the sky is blue needs explaining that it is due to Rayleigh scattering. Citing the sources which explain why it is blue would be valuable to some readers.
Readers come from different backgrounds and with different knowledges, cultures and experiences. It would be inappropriate to assume everyone's knowledge is the same.
Before making a statement, no matter how trivial or "obvious", make sure that it could not be misinterpreted or challenged.
The question is not whether readers can or can not be expected to have knowledge of a certain fact, but whether the fact in question is a relevant point of debate in the expert literature on the article topic.
If there is any doubt about the claim addressed in the relevant literature, you should cite it. If the relevant expert literature does not bother to address a point (e.g. because it falls under WP:FRINGE, and its mere mention, if only to debunk it, would lend it WP:UNDUE relevance), it is likely that the corresponding Wikipedia article shouldn't, either.
If it really is common knowledge, it really isn't that difficult to source. For example, this source supports both the idea that the sky is blue, and that the blueness of the sky is common knowledge:
One can go on to mention that the poet Robert Service says "while the blue sky bends above/You've got nearly all that matters". Songwriter Irving Berlin wrote of "Blue Skies smiling at me," airmen fly into the wild blue yonder. And one can, of course, cite Rayleigh's paper, "On the scattering of light by small particles," Philosophical Magazine 41, 275: pp. 447–451.
When a statement that you feel to be obvious is challenged, try to think of a person (such as a person in a foreign country) to whom the statement might not be obvious, or a situation in which your obvious statement might be wrong.
The sky actually appears to be blue less than half the time. Some conditions under which the sky may not appear blue: