From the _Zork _“Invisiclues” booklet, on the subject of things you simply can’t do:
How do I go over the falls without killing myself?
_Why not ask, “How do I cut off my head without killing myself?” _
Pretty much the only source for that quote; Google Books doesn’t seem to have the “Invisiclues” line scanned. (I wonder how the invisible-ink fares over decades.)