[Cicero]

When does something cease to be uncommon? I took a plagiarism quiz and missed a question. I marked it as "common" knowledge. So is there some kind of update I am missing as to what is common knowledge today? How am I supposed to know what everyone knows, as opposed to what they don't know?

Shall I just give reference to everything just assuming they know nothing? Just to be sure?

The question still stands-when does knowledge become common? Who is this "everyone" that knows this knowledge?