I love it when Canadians talk about "taking the 5th", ie. invoking their "Fifth Amendment rights." We don't abide by the American Constitution. We do, however, have a Constitutional Right against self incrimination. You cannot be compelled to bear witness against yourself. In practice this means that you cannot be compelled to take the witness stand in a legal case against you, however, if you're dumb enough to take the stand then you must answer all questions put to you, by either Defence or The Crown, truthfully. You can't simply decide that answering questions from The Crown isn't in your best interests. That way lies criminal contempt charges.
Even laws benefit from good marketing. "I plead the fifth" is just a way better slogan than "Sorry, but I need to invoke my rights against self incrimination, eh."