| Knots |
| Posers |
| Puzzles |
| Riddles |
| Sphinxes |
| Mysteries |
| Hard nuts |
| Puzzlers. |
| Perplexers |
| Conundrums |
| Puzzlements |
| Closed books |
| __ paradoxes |
| Brain-teasers |
| Real puzzlers |
| Puzzling things |
| Obscure riddles |
| Puzzling posers |
| Inscrutable ones. |
| Inscrutable types |
| Baffling problems |
| Hard nuts to crack |
| Perplexing puzzles |
| Inscrutable people |
| Tough nuts to crack |
| W.W. II code machines |
| They're hard to figure |
| Contradictory characters |
| Inexplicable occurrences |
| Causes of head-scratching |
| They're hard to figure out |