The Pebbles game has two main challenges.
The first challenges are the NxN -1 sliding puzzles, including the 8-puzzle, 15-puzzle, 24-puzzle, and so on. These puzzles have classic modes, an expert mode, and time-based challenges. The expert and time modes are very smooth and fun/engaging!
The second challenge is a variation of the 15-puzzle that requires the player to either flip two tiles or build a circle of tiles that would rotate synchronously. This is a challenging puzzle if you want to get three stars! In the first release, 16 stages are released. More stages will be released in the future - let me know if you like the game.
The game is entirely free.