I believed it hardest puzzle of the lot.

That's when they get so dreadfully mixed, and it's a hard puzzle to put them together again.

Using letters instead of numbers creates a harder puzzle.

If a team was able to solve all four puzzles on the board, they were shown a fifth, harder puzzle.

Browse the article What's the world's hardest puzzle?

Why, your expression makes it a harder puzzle than before.

That was the hardest puzzle I had to solve throughout this episode.

The Friday puzzle, however, is known as one of the harder puzzles of the week and often includes uncommon names.

Perhaps the hard puzzles are meant to counterbalance the soft-headed story.

Players generally must complete each puzzle in order to progress to the next one, but the game includes optional harder puzzles.