The day when a Frenchman switches from the formality of vous to the familiarity of tu is a day to be taken seriously. It is an unmistakable signal that he has decided - after weeks or months or sometimes years - that he likes you. It would be chulish and unfriendly of you not to return the compliment. And so, just when you are at last feeling comfortable with vous and all the plurals that go with it, you are thrust headlong in to the singular world of tu.