| 2024-08-22 | Tough nut to crack |
| 2024-07-23 | Cipher machine of W.W. II |
| 2024-02-12 | Riddle |
| 2022-09-07 | Very secretive sort |
| 2021-10-24 | Personality that's hard to read |
| 2021-08-15 | Monthly publication of the National Puzzlers' League, with "The" |
| 2020-11-30 | Conundrum |
| 2020-09-30 | Puzzle |
| 2020-05-29 | Tough nut to crack |
| 2019-04-04 | Puzzle |
| 2018-02-04 | Mystery |
| 2017-10-21 | The Mona Lisa's smile, e.g. |
| 2017-03-26 | Conundrum |
| 2016-10-22 | Tough nut to crack |
| 2016-02-14 | Perplexity |
| 2015-11-18 | Nazi cipher machine broken by the Allies |
| 2015-08-19 | Mind-boggler |
| 2015-07-17 | "The Imitation Game" machine |
| 2014-09-23 | Riddle |
| 2013-01-10 | It's a mystery |
| 2012-02-18 | "I'm a walking, talking ___": Larry David |
| 2010-02-21 | Appropriately named monthly of the National Puzzlers' League, with "The" |
| 2010-01-30 | Head-scratcher |
| 2009-11-24 | Riddle |
| 2009-04-06 | Riddle |
| 2007-09-01 | Knot |
| 2007-06-05 | Conundrum |
| 2007-04-10 | Riddle |
| 2007-03-04 | Head-scratcher |
| 2005-12-11 | It's not clear |
| 2005-02-19 | It's hard to understand |
| 2003-07-20 | "Hmm?" inducer |
| 2003-02-15 | Stumper |
| 2002-11-08 | Mind-boggler |
| 2002-08-01 | Poser |
| 2002-03-12 | Riddle |
| 2001-11-21 | Stumper |
| 2001-10-25 | It's mind-boggling |
| 2001-10-15 | Perplexer |
| 2001-06-26 | Something inexplicable |
| 1999-10-11 | Mystery |
| 1999-07-14 | Complete baffler |
| 1998-04-14 | Mystery |
| 1997-12-24 | Elgar's "___ Variations" |
| 1997-10-19 | Poser |
| 1997-08-03 | Sphinx |
| 1997-05-20 | Puzzle |
| 1995-08-28 | Elgar's "____ Variations" |
| 1995-07-13 | Perplexity |
| 1995-05-02 | Puzzle |
| 1993-12-31 | Mystery |