Another interesting mechanism is what ChessTempo.com uses. Players and puzzles both have a Elo-like dynamic ranking. When a player beats a puzzle, the player's

Another interesting mechanism is what ChessTempo.com uses. Players and puzzles both have a Elo-like dynamic ranking. When a player beats a puzzle, the player's rank increases and the puzzle's rank decreases. This makes it easy to, over time, present players with puzzles that are of appropriate difficulty.