Everyone knows, they say

Common knowledge is a complex thing. Defining it is difficult, and applying specific bits of information to belong to that definition, once you find one, is even harder. What can you assume people already know? Should you aim high or low? The pros and cons can be heavy stuff, for either.

If you assume a lot of knowledge on the part of your audience, you can cause people to learn by making them look things up. Or make them angry that you are being pretentious and arrogant, and throw down your writing in disgust.

And writing down, explaining things overmuch, might make sure that everyone understands, but is no better because then you're talking down to the rest who already know the information, or boring them. This comes up when you write a paper on a specialized subject, or just when you're talking to people you don't know well.

"Everyone knows science fiction is dying. Everyone knows science fiction is doing just fine, even thriving. Everyone knows the future of science fiction is in debate. Everyone knows the future of science fiction has shifted into YA while most people weren't paying attention."

You can find all of this as "everyone knows." See the problem?

And don't forget the dreaded "They say". English teachers hate to see it, don't they? Not just the teachers, though. You yell at your friend, "Who says? What's your source?" And in reply, you recieve only the nebulous "I don't know, everyone"- the ghostly archetype haunting the societal zeitgeist. Frustrating.