The InvisiPen™ was a yellow marker that shipped with Infocom InvisiClues hint books. The books themselves were mostly blank; they listed only the questions, and then a blank area beneath each question that you would color in with the InvisiPen, thus revealing the hints.

The hints were printed in some kind of invisible/white ink, on white pages. The yellow marker, when dragged across the white ink, would illuminate the letters in a white-on-yellow area that seemed to fade after only a few months, mostly due to the moisture of the marker sullying the paper of the pages.

The pens themsevles were inevitably lost by adolescent Infocom gamers, and a variety of replacements were used. Sometimes flourescent yellow highlighters would work; sometimes it would be steam from an iron; blacklight worked sometimes, depending on the condition of the book and whether the hint area had already been colored in by the InvisiPen.

An example of a typical hint, in this case for the original Zork, looked like this, prior to the intervention of the InvisiPen:

               Q4. Is the nest useful for anything?
               ___________________________________________
              |                                           |
           A: |                                           |
              |___________________________________________|

               ___________________________________________
              |                                           |
           A: |                                           |
              |___________________________________________|

               ___________________________________________
              |                                           |
           A: |                                           |
              |___________________________________________|

However, after running the InvisiPen back and forth over the above blank spaces a few times, you'd find the answers you sought, although most of the answers were pretty useless and were written in the beloved acerbic style that Infocom text adventure games came to be known for.

The freshly InvisiPen'd hints looks like this (imagine the example below is awash in still-drying yellow ink), at least until they faded four or five months later:

               Q4. Is the nest useful for anything?
               ___________________________________________
              |                                           |
           A: | In China you might make bird's nest soup. |
              |___________________________________________|

               ___________________________________________
              |                                           |
           A: | This is not China.                        |
              |___________________________________________|

               ___________________________________________
              |                                           |
           A: | In other words, no.                       |
              |___________________________________________|

As far as I've been able to ascertain, no Infocom-released hint books complete with InvisiPen exist to this day. I couldn't find a single one on eBay or any other classic gaming sites. Even The Lost Treasures of Infocom didn't come with this style of hint book; it was just plain old, boring text. No heady marker fumes or bits of yellow bleeding through to the next page. It was then that I knew that Infocom was no more.

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