The game seriously needs some kind of hint in the higher levels, when difficulty takes away most of the fun from the game. Even the last level feels like a picnic when compared to the likes of levels 32, 38 and 39.
There are also a couple of glitches: Sometimes the game doesn't recognize when you have clicked on a piece. More than twice I found that two pieces seem to link their movements, especially when you superimpose them, or when the game superimposes them after you drag one piece too fast and then it is sent to the margins, forcing you to move a piece you don't want in order to break their link.
Otherwise, an ingenous idea, but I'll deduct a half star because of difficulty for the sake of being difficult.