The downloadable content features four new, extra challenging puzzle levels.

Because the reader obtains clues in the same way as the protagonist detective, the book becomes an intellectually challenging puzzle.

Understanding that the paper was certain to generate controversy, it was presented to readers in the context of a challenging puzzle.

To get the maximum benefit out of brain-training exercises, scientists suggest that you do them regularly and tackle increasingly challenging puzzles over time.

Modern computers have been used to search for similar patterns and more complex variants, and it has been published as a challenging puzzle in a peer-reviewed academic journal in 1994.