(Man): Dr. Brighton’s Biology quizzes are short and tricky.
(Woman): Aren’t they, though?
[...] What troubles me is the "though" part. I think the "aren't they" is an agreement [...]
I agree with Gustavo's answer, as well as with his point that your new question here, Cecilia, should have begun a new thread, insofar as it introduces a new topic -- one which is very interesting. I can't recall anyone on GE ever having asked about this special use of "though" in tag questions before.
I thought that the O.E.D. (The Oxford English Dictionary) might address this construction in its entry for "though," and it does. It classifies this use of "though" as colloquial and gives the following definition: "used as an intensive after a question or emphatic statement: indeed, truly." Here one O.E.D. example:
- ‘What a sad story!’ said Maria. ‘Isn't it, though?’ (G. Vidal, City & Pillar, 1948)
You are right, Cecilia, that this type of tag question expresses agreement. It is as if the person responding with the tag pretends that he or she has made the statement with which he or she is agreeing. Other things we sometimes say in these contexts include "It's so true!" (colloquial) and "I couldn't agree more."