Closed book and Question mark

Closed book noun - Something hard to understand or explain.

Question mark and closed book are semantically related. In some cases you can use "Question mark" instead a noun phrase "Closed book".

Question mark noun - A punctuation mark (?) placed at the end of a sentence to indicate a question.

Closed book and question mark are semantically related. Sometimes you can use "Closed book" instead a noun phrase "Question mark".