The pay-per-hint service eventually led to the development of InvisiClues: books with hints, maps, clues, and solutions for puzzles in the games. The answers to the puzzles were printed in invisible ink that only became visible when rubbed with a special marker that was provided with each book. Usually, two or more answers were given for each question that a gamer might have. The first answer would provide a subtle hint, the second a less subtle hint, and so forth until the last one gave an explicit walkthrough. Gamers could thus reveal only the hints that they needed to have to play the game. To prevent the mere questions (printed in normal ink) from giving away too much information about the game, a certain number of misleading fake questions were included in every InvisiClues book. Answers to these questions would start by giving misleading or impossible to carry out answers, before the final answer revealed that the question was a fake (and usually admonishing the player that revealing random clues from the book would spoil their enjoyment of the game). The InvisiClues books were regularly ranked in near the top of best seller lists for computer books.