Zork

> What are InvisiClues?
InvisiClues for this question (move your mouse over the black-on-black text one line at a time):

? InvisiClues were hint booklets sold by Infocom to help players solve puzzles in their interactive fiction computer games. Questions relating to the game were printed in the book, for example, the InvisiClues for Zork I contained the question "How can I kill the songbird?" An "empty" box was located below or following the text. The answer to each question was revealed by using a special highlighter-like marker that came with the book; this marker contained a special solution that made the previously hidden answers visible. Over time, the ink degraded and the text reverted to invisibility.

? To discourage players from accidentally learning what awaited by reading all the questions, each booklet contained a number of plausible-sounding "fake" questions. Revealing these answers usually resulted in a mild scolding. Several "non-puzzles" also had questions, such as the songbird example used above. The answer to these was usually a tersely-worded statement saying "You can't do that", often followed by one or more items reading "This space intentionally left blank."