Whether you’re interested in sinking a lot of time into a game or just picking up something that can occupy you in the waiting room at the doctor’s office, Flow is worth a download. The game is a simple twist on connect-the-dots: Players are faced with a grid of different-colored pairs of dots. The goal is to connect the right dots while filling up every square on the grid. It sounds easy — and is, at least in early levels — but quickly becomes a brain-twisting exercise in spatial thinking. The app has a good number of levels, even in the free version, although users who aren’t interested in spending money on the app do have to deal with ads. But even that minor interruption doesn’t detract much from this delightful brainteaser. Free, with in-app purchases, for iOS and Android devices.