What's a good iPhone game to keep my brain sharp during downtime, something that's actually challenging but not gonna make me wanna throw my phone?

Look, I need something to do while I'm waiting for clients, and those brain-dead matching games just don't cut it. This Word Solitaire-Unscramble Puz game is actually kinda fun. It's a word game, so it gets the ol' gray matter working, but it's not so hard that you're ready to chuck your phone across the room. I've been playing it for about 20 minutes a day, and I feel like it actually helps me stay focused. Plus, it's a decent way to kill time without feeling like I'm wasting it completely. Bottom line, it's worth a download if you're looking for a game that's challenging without being infuriating. Keeps the mind sharp, which is always a good thing as you get older. And hey, it's cheaper than hiring a personal trainer for your brain.