| inscrutableness(difficulty, question, secret, puzzle, perplexity) |
| iron(adjective, mean, hard, tough, harsh) |
| mean(adjective, hard, tough, harsh, unfeeling) |
| mindboggler(difficulty, question, secret, puzzle, perplexity) |
| mulish(adjective, mean, hard, harsh, inflexible) |
| mystery(noun, difficulty, question, problem, secret) |
| obstinate(adjective, mean, hard, harsh, inflexible) |
| occult(difficulty, question, secret, puzzle, perplexity) |
| oracle(difficulty, question, secret, puzzle, obscurity) |
| perverse(adjective, mean, hard, tough, harsh) |
| pigheaded(adjective, mean, hard, harsh, inflexible) |
| poser(difficulty, question, secret, puzzle, obscurity) |
| puzzle(difficulty, question, problem, secret, obscurity) |
| puzzler(difficulty, question, problem, puzzle, perplexity) |
| question(difficulty, secret, puzzle, obscurity, knot) |
| rebus(difficulty, question, secret, puzzle, perplexity) |
| relentless(adjective, mean, hard, tough, harsh) |
| riddle(noun, difficulty, question, secret, puzzle) |
| rigid(adjective, mean, hard, severe, stiff) |
| rune(difficulty, question, secret, puzzle, perplexity) |