If your professor announces that your next exam will be open-book (meaning, unlike in normal circumstances, you will be allowed to open your books / notes / math formulas / etc. while you take the exam), do you look at it as an extra challenge? (Because if your professor were sadistic enough, open-book exams mean the same as saying "I'll give you an exam so hard that you won't get a passing grade... even if you've got your books open." )