In one famous case, a collection of enigmas, cryptograms, quandaries, rebuses, cruxes, and conundrums are grouped and named according to difficulty. The groups are (from easiest to most difficult) CRYSTAL, LEAF, SAND, WATER, METAL, and FIRE.
Each category contains a sequence of eight questions of increasing difficulty. Therefore, the eighth question of the FIRE set is considered by many to be the most difficult riddle of the collection, and possibly the most difficult question in existence.
The phrase "Fire 9" is a term used to describe a hypothetical riddle or question whose solution is exceedingly difficult to derive or comprehend.
Person B: "You may as well try to find an end for pi. Give it up man, it's an enigma, a Fire 9."