I love the app but I do have a complaint. Occasionally, a puzzle piece will scatter to an unreachable location. You can restart the puzzle and you may be successful in keeping all of the pieces on the reachable surface on the restart. However, you may not immediately realize a piece is missing until you are almost finished working the puzzle-which is frustrating when working a puzzle of some difficulty. Its disheartening to work for a couple of hours only to realize your work was for naught because you came up one piece short. :(