[nkmnz]

Not of thin air, but of what you've studied so far. Of course some (many) teachers are imbeciles and just use the same exam they've used in the previous year, even if they haven't covered all the topics yet. But in general, it's a very good idea to have a small question that tests for real understanding, i.e. applying your knowledge to a problem not previously presented, versus mindless reproduction of memorized facts.