Puzzler (noun)

A person or thing that puzzles or is difficult to understand or explain.

Origin:

Late 16th century: from puzzle + -er.

Examples:

1. The math problem was a real puzzler for the students.
2. The detective couldn't solve the puzzler of the crime.
3. The crossword puzzles were a real puzzler for her.
4. The new technology was a real puzzler for the older generation.
5. The history of the ancient civilization was a great puzzler for the archaeologist.