Monument Valley is like one those images brought to life. It's a puzzle game that tasks you with guiding little princess Ida through a series of levels, packed full of impossible geography and optical illusions.
You manipulate parts of the environment to make new routes that'll see you on your way.
It's a difficult concept to explain in words - the image at the top of this post shows you how it works more effectively. What's remarkable is how quickly your brain adjusts to this impossible logic. It won't be long before you're prodding and swiping your way to success.
That's partly because it's so easy to control - you simply touch the screen where you want your character to move, and slide bits of the scenery around as needed. Factor in the soothing music and beautiful graphics and you have an app that's actually more relaxing than it is mind-bending.
It's also really short. It took me about an hour to work through all 10 chapters. Whether £2.49 for an hour's amusement constitutes good value is debatable - but for my money, the app is so intelligent, well-crafted and memorable that it justifies the asking price.
You can download Monument Valley from the Apple App Store.