What's a good iOS game for a quick mental exercise during breaks between research papers?

I've found Number Master: Ten Pair Puzzle to be a surprisingly effective way to clear my head between writing sessions. The premise is straightforward – you pair numbers that add up to ten – but the algorithm is surprisingly elegant in how it generates the puzzles. It's intellectually satisfying to find the solutions, and the game's simplicity means I can pick it up and put it down easily, which is essential when I'm trying to maintain focus on my research. It's a novel approach to an old problem, really – providing a quick, accessible mental workout. I could see this being pedagogically sound for students as well, as a means of improving quick arithmetic skills. Plus, it's available on my iPhone, so it's always accessible.