This is one of the best puzzle games I've ever played, and I've played many. The puzzles teach you the mechanics, and many of them stretch your brain but there is always a solution. Only one element of one puzzle is overly timing-based and perhaps unfair based on what you have learned. But the difficulty level was generally just right. Undo is very welcome, but buggy. If you undo a thrown block, and it glitches making the block go away, just try to move, then undo again, then the block will be thrown again and you can pick it up. Given that some puzzles ARE timing based, I understand why undo was difficult to get right. Even so, still worth 5 stars due to the excellent puzzle design. Graphics and sound design are great, too.