Riddle title: The new moon is hidden in the sky (five-character common expressions)
Riddle analysis: "Kong" means "eight", and "new moon" refers to the moon that appears at the beginning of each month in the lunar calendar The curved moon resembles the shape of "俿", so "crescent moon hidden" means "leaving it out". This common expression refers to something that has no clue or clues.
Answer to the riddle: Not a single character