Grammatically together

Overheard: A discussion on subject-predicate agreement:

He: Why would you have "the pair of cats is sitting" but then "the pair of cats were sitting"?
She: Because in between they had a fight?*


* The rule they teach you in grammar school is that the collective noun takes a singular verb. But the real rule has to do with the sense of the sentence - if the writer wants to emphasize the cats rather than the fact that they're a pair, the plural verb is correct.