Of course, people's vocabulary and knowledge differ anyway. This may be the very first clue in the puzzle (1 Across), but crossword authors and editors don't automatically make the first clue the easiest one. The first answer you should write in a crossword should ideally be one whose clue you are sure of. Once you have correctly identified the theme, the remaining theme answers should be much easier for you to puzzle out. When you have filled in the first long answer, take another look at the title you should be able to make a guess now as to what the theme is. The title is there to give you a hint about the theme of the puzzle-the subject matter or the common element among the longest answers.
You should always look at the puzzle title before doing anything else. How to Play Stan Newman's Sunday Crossword
