Obviously "the sky is blue" should not be deleted. But it's a rather bad example for your purpose, as I recently noted on a talk page. In the first place, the sky is "not" always blue, and therefore this is a "fact" that is not really quite true. In the second place, as is so often the case of things that "can't be sourced because they're just common knowledge," it is ''very'' easily sourced: "the blue sky is so commonplace that it is taken for granted" {{cite book|title=A Field Guide to the Atmosphere|first=Vincent J.| last=Schaefer|coauthors=John A. Day|year=1998|publisher=Houghton Mifflin Field Guides|id=ISBN 0395976316}} I also found: 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 (song)|Blue Skies]] smiling at me," airmen fly into the [[The U.S. Air Force (song)|wild blue yonder]]. {{cite book|title=Collected Poems of Robert Service|first=Robert| last=Service|year=1940|publisher=G. P. Putnam's Sons|id=ISBN 0-399-15015-3}},</ref> For the sky _not_ always being blue: Matthew 16:2, Jesus says to the Pharisees "When it is evening, ye say, It will be fair weather: for the sky is red." "At twilight, salmon reds, oranges, purples, white-yellows, and many shades of blue can be seen." {{cite book|title=Light and Colour in the Outdoors|first=M. G. J.| last=Minnaert|origyear=1974|year=1993|publisher=Springer-Verlag| id=ISBN 0-387-97935-2}} p. 295 And songwriter [[Oscar Hammerstein II|Oscar Hammerstein]]'s famously wrote of "when the sky is a bright canary yellow."<ref>{{cite book| title=American Musical|first=Marc|last=Bauch|publisher=Tectum Verlag| id=ISBN 382888458X}} </ref> It took me less than ten minutes to turn up the "serious" sources (Schaeffer and Minnaert) another fifteen to find the rest. If something is really a commonly known fact, it is just not that hard to source. And the effort to do so _often_ yields dividends. This is not to say that requests for citations can't be abused. But, if I am an editor, and I inserted something without a source because I didn't really think it needed one, and someone marked it "citation needed," my response would be to source it. This has happened to me, and it's what I've done. Except in one case... where I couldn't find one... and so I removed the item myself.