Seinfeld’s George Costanza was once invited “up for coffee” at the end of a romantic evening, and refused: caffeine would keep him awake, he explained to his perplexed date. Later, aghast, he realised: “coffee doesn’t mean coffee! Coffee means sex!”

Well, indeed – but few people, if they are wise, will baldly suggest the sex. A little ambiguity is called for. Now game theorists – masters of the mathematisation of human interaction – are beginning to produce rational models of deliberate vagueness.

Andreas Blume and Oliver Board, two economists at the University of Pittsburgh, offer up just such a model. They point out that perhaps it is too much trouble to be specific, as with a business contract offering a fee plus “reasonable expenses”. This isn’t always the reason. “Coffee” has two syllables, “sex” has only one, and surely George’s date could have made her intentions plainer without much effort.