InvisiClues were hint booklets sold by Infocom to help players solve puzzles in their interactive fiction computer games.
[1] Before Infocom's games exploded in popularity, players could request hints by mail and receive a type-written sheet in response.
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" or, on occasion, showed false clues such as "How Do I get off the roof of the House?"
Even the answers to real questions began with vague hints, so a player could choose to stop short of getting explicit solutions to the puzzles.
Typically, either all the answers are printed normally on the site or the user must "highlight" a section by clicking and dragging the mouse to reveal the hints.