Yeah, I get this in my work, too. Teachers like to chortle about catching plagiarizers or failing someone who, say, hasn't done half the work, but actually sitting down with them and going through it suuuuuuucks. Whether the answer is tears, belligerence, or just a complete inability to grasp what the problem is, it almost always requires a few moments in a quiet office after. Doing it over email is usually (but not always) easier, but it's seldom fun.