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There are a lot of things that "everybody knows." Everybody knows that Everest is the tallest mountain on Earth, that 2+2=4, that most people have two eyes—and a lot of other things. If I were to go on, it would get tedious very fast, because, after all, these are things that everybody knows.

But there are also a lot of other things that "everybody knows," except that not everybody agrees that everybody knows them. For example, everybody knows not only that there has been significant global warming recently, but also that human beings caused this by burning fossil fuels. We know that evolution is as solidly proven as most of the rest of science, and that intelligent design isn't science at all; that Iraq never had any weapons of mass destruction (after they destroyed them); and that the U.S. government had nothing to do with the destruction of the World Trade Center. Except that, for each of these things "we all know," significant minorities insist that they're false.