Kubic has been on my mind for a while.  Description from the App Store:
kubic is a unique optical illusion puzzle game based on M.C. Escher's art, impossible objects and other geometric designs. The object is to construct the goal configuration from a number of pieces.Presented in a simple, clean design and minimalistic art, kubic is a premium game with 60 escher-esque puzzles.When you click on a piece, it pops to the front-most layer, and may overlap other pieces. When you combine the pieces, they do not merge. If there are extra pieces, you may hide them behind the others. If you are stuck on a puzzle you can simply skip it and re-visit at a later time. Rest assured, all puzzles can be solved.If you truly get stuck, there is a in-game hint system. You can use it to get started or even get the whole puzzle solved.   I'm not sure how many levels I solved. I played this during the summer. I'm guessing about one-third.  They get pretty tough!

https://itunes.apple.com/app/kubic/id1115010672"ls=1&mt=8

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