Players who are bluffing will be reluctant to exhale their cigarette smoke in a conspicuous manner. Remember, bluffers try to do nothing to bring attention to themselves and promote a call. The exhalation is an unaware tell that's just about as likely to be valid for strong players as for weak players. It will occur more times per hour, however, in smaller games. Since this tell is only 60% reliable, keep in mind that the opposite of what you expect will frequently happen. Occasionally you'll see a cloud of smoke accompany a bluff, even though your clue should weigh in the opposite direction.
