Closed book and Tough nut to crack

Closed book noun - Something hard to understand or explain.

Tough nut to crack and closed book are semantically related in puzzle topic. In some cases you can use "Tough nut to crack" instead a noun phrase "Closed book", when it comes to topics like mystery, brain-teaser.

Closed book and tough nut to crack are semantically related in puzzle topic. You can use "Closed book" instead a noun phrase "Tough nut to crack", if it concerns topics such as mystery, brain-teaser.