It's a tough question, deciding what is common knowledge and what requires attribution (citation/reference in APA format).
I use an example from the university's plagiarism tutorial to demonstrate the difficulty that we (as scholars) can face. The tutorial gives an example of common knowledge facts; however, I am willing to bet that most of us aren't able to correctly answer one of the questions posed as an example. In fact, I rarely meet folk between the age of 10 and 40 that can correctly answer the example question...
![[help]](/images/question.gif "[help]")
)![[Attribution]](http://creativecommons.org/images/public/somerights20.gif "[Attribution]")