This tag is for arguments that produce an inconsistency with common sense.
Paradoxes are interesting, because they often show places where our intuition or logic fail.
Take the Liar Paradox:
The first sentence in this essay is a lie. There is something odd about saying so, as has been known since ancient times. To see why, remember that all lies are untrue. Is the first sentence true? If it is, then it is a lie, and so it is not true. Conversely, suppose that it is not true. As we (viz., the authors) have said it, presumably with the intention of you believing it when it is not true, it is a lie. But then it is true! ... Lying is a complicated matter, but what's puzzling about sentences like the first one of this essay isn't essentially tied to intentions, social norms, or anything like that. Rather, it seems to have something to do with truth, or at least, some semantic notion related to truth.
In this case, a single sentence leads immediately to questions about how we consider truth.