What's a good iOS game for a quick mental refresh during breaks, something that isn't too taxing but still keeps the mind engaged?

I've found WordsSoup - Word Search Puzzle to be a surprisingly elegant solution for those moments when I need a mental break but don't want to completely disengage. The algorithm is surprisingly elegant; the puzzles are well-constructed, and the interface is clean. It's pedagogically sound, in that it reinforces vocabulary without feeling like rote learning. The cross-platform compatibility is a plus, allowing me to pick up where I left off on my iPad or iPhone. I can envision using this in a classroom setting as a quick vocabulary exercise. It's a novel approach to an old problem, and the well-researched design makes it intellectually satisfying. I think it would also be helpful for managing citations, given the mental break it provides.