Wikipedia:You do need to cite that the sky is blue
 | This essay contains the advice or opinions of one or more Wikipedia users. Essays may represent common ideas, or ideas that many users would not support. They are not rules. Think carefully about what they say before following them. |
It might be simple for us not to cite things we think are common knowledge. However, sometimes you do need to cite it, even if it's common knowledge. Common knowledge depends on conditions or sometimes, it is wrong. In this example, the sky is not always blue, but it depends on the weather conditions, like the images on the right.
However, you don't need to cite every thing in Wikipedia. For example, you don't need to cite that an article is an article, or every period, just because it's the grammar of English.