15challenge by nuvolabs
This app contains an impossible version of the old 15 puzzle (that is, it starts in a mathematically-proved unsolvable mode). Make your friends try and re-try in the unsolvable mode, they will never succeed in solving the puzzle. When it’s your turn, press the invisible (hidden) button and you will be able to solve it easily.
It works like this:
- by pushing the “shuffle” button it starts in the unsolvable mode;
- by pushing at the same time the “shuffle” button and the invisible button on the left, it starts in the solvable mode. A small fly on the right top corner of the screen indicates that the normal mode is active.
Developer Website: http://www.nuvolabs.net/apps-1/15challenge