Common knowledge is not a category of facts. It is a relationship between a fact and an audience, which is why the same sentence can need a citation in a first-year essay and not in a graduate seminar, in the same building, in the same week.

That is why "is this common knowledge?" never resolves by staring harder at the fact. It resolves by asking who you are writing for, and there are established tests for working that out.

The four tests

Yale's Poorvu Center sets out the standard ones in its guidance on common knowledge, and between them they cover most real cases:

The educated reader test. Would most educated people already know this? Water is H₂O passes. The precise bond angle does not.
The easy lookup test. Can it be found in a general reference work, an encyclopedia or a dictionary?
The five sources rule. If the fact appears uncited in five or more reliable sources, it is treated as community property rather than any author's contribution.
The ownership test. Can any individual fairly claim to have discovered or originated this? If yes, cite them.

The tests agree most of the time. When they disagree, the ownership test is the one to trust, because it targets what citation is actually for: credit for someone's specific contribution.

How to actually use the five sources rule

It is the most practical of the four and the most often misapplied. The rule is that the fact appears uncited in five or more reliable sources. Five sources that all cite the same original study prove the opposite of common knowledge: they prove everyone knows exactly whose finding it is.

So when you check, look at what the sources do with the fact. If five textbooks state it flatly as background, you have common knowledge. If five papers state it and each one attaches the same name and year, that name and year belong in your sentence too.

The same fact, two different answers

Yale's own examples make the audience point better than an explanation does. Within psychology, that chimpanzees recognise themselves in a mirror is common knowledge and needs no citation. Within literature, that James Joyce is a major modernist author needs none either.

Move either sentence into a first-year composition essay for a general reader and the calculation shifts, because the audience shifts. Move to the interpretation rather than the fact, and it shifts again: what mirror self-recognition implies about models of consciousness is somebody's argument, and arguments always carry citations.

Yale also flags a caveat that catches people in the sciences, where new knowledge becomes old knowledge quickly. A finding that needed a citation five years ago may be background now, and the only reliable way to know where that line currently sits in your field is to read what recent papers in it treat as needing support.

What is never common knowledge

Four categories, regardless of audience:

Specific numbers from specific studies. "Most students report high stress during exams" may be common knowledge. "68% of students reported high stress" is somebody's data, and the moment you use their figure you are using their work.

Interpretations and arguments. The fact that a war began in a given year is common knowledge. Why it began is a position somebody holds, and other people disagree with it.

Anything contested. If experts in the field are arguing about it, it fails the test by definition, because common knowledge is supposed to be undisputed as well as widely known.

Original phrasing. The fact may be free. The sentence somebody wrote about it is not, which brings us to the trap.

The wording trap

This is where most people who understand common knowledge still get flagged.

You establish, correctly, that a fact needs no citation. You then reproduce the source's sentence about that fact, or a lightly reworded version of it, because that sentence was well written and it was in front of you. The fact was common knowledge. The prose was not.

A checker will still highlight that passage, and it is right to. What you have produced is a patchwrite that happens to be about an uncontroversial fact, and the fact being free does not make the sentence yours.

Common knowledge frees you from citing the idea, never from writing your own sentence, so the fix is the same as any other close rewrite: close the source and write the fact from what you know. If you find you cannot, that is worth noticing, because it usually means you were leaning on the phrasing rather than the fact.

Worked examples

Rules of thumb are easier to apply against real sentences. Assume an undergraduate essay for a general reader:

"Water freezes at 0°C at sea level." No citation. Reference-work material, uncited everywhere, nobody's discovery to claim.
"The Berlin Wall fell in 1989." No citation. A widely known date.
"The Berlin Wall fell partly because a spokesman misread a note at a press conference." Cite it. A specific historical account, and one that historians present with varying emphasis.
"Exercise improves mood." No citation as general background.
"Thirty minutes of moderate exercise three times a week reduced depressive symptoms by 47%." Cite it. That is a study, and the number is the giveaway.
"Shakespeare wrote Hamlet." No citation.
"Hamlet's delay reflects an early modern anxiety about the reliability of ghosts." Cite it. That is a reading somebody published, and other critics argue otherwise.

The pattern across all seven: the moment a sentence carries a number, a mechanism, or an interpretation, it stops being background and starts being somebody's work.

It applies outside coursework too

The same logic governs writing published on the web, with a different penalty attached. No integrity board reviews a blog post, but a reader who recognises an uncited statistic loses confidence in everything else on the page, and a competitor whose research you used without attribution will notice.

The practical standard for professional writing is looser on background and stricter on data. Nobody links a source for "most people check their phone in the morning." Everybody should link one for "people check their phones 144 times a day," because that figure came from a specific piece of research, and a reader who wants to evaluate it needs to see whose it was.

When you genuinely cannot tell

Cite it. The cost is asymmetric and it is not close.

An unnecessary citation makes you look slightly over-careful. A missing one on something that turned out to be a specific author's contribution is a misconduct question. Nobody has ever been penalised for citing a fact that did not require it.

Two things temper that. Yale tells students to ask their instructor when in doubt, and they mean it: the person setting your assignment is the person defining your audience, so one email settles what a rule of thumb cannot. The other is that a paper citing every single sentence reads as though you have no view of your own. If your draft looks like that, the problem is usually not your citation habits. It is that you are reporting sources instead of arguing.

A practical order of operations

When a sentence makes you hesitate, run it in this order and stop at the first clear answer:

Is this someone's specific finding, number, or argument? Cite it. Done.
Would my actual reader, in this course, already know it? If clearly yes, no citation needed.
Does it appear uncited in several reliable sources? If yes, treat it as common knowledge.
Still unsure? Cite it, and ask your instructor before the next assignment.
Whatever you concluded, is the sentence in front of you your own? If it came from a source, rewrite it regardless of the citation decision.

That last step is the one people skip, and it is the one that produces highlights in a report written by someone who did everything else right.