Closed book and Rebus

Closed book noun - Something hard to understand or explain.

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

Rebus noun - A puzzle where you decode a message consisting of pictures representing syllables and words.

Closed book and rebus are semantically related in puzzle topic. You can use "Closed book" instead a noun "Rebus", if it concerns topics such as mystery, brain-teaser.