The basic idea of Flow Free is that you're presented with a black grid containing multiple colored dots, two dots of each color. You need to connect dots of the same color by dragging a path between them, and end up with every pair of dots connected in a way that fills every piece of the grid. It's a simple premise, and the challenge level scales well from simple to total brain stumper.
Puzzles in Flow Free are arranged based on grid size. From the main menu, you can select any size grid without having to have completed the previous sizes, allowing you to jump right into whatever level of challenge you are in the mood for. This approach makes the game more accessible to casual players, and allows the easier earlier levels to be skipped if they don't provide enough difficulty, but on the other hand, you do lose out on any sense of progression you might get otherwise.
Lacking a progression map may seem like it would make the game less addictive, but if it does, I didn't notice. This is one of those "Just one more level" games that is difficult to put down. It loads fast and remembers where you left off, so it's a quick go-to when waiting in line, waiting for a download, waiting for a text reply, or just plain waiting. I found myself going through the lists on my phone until I'd completed every puzzle, even the larger ones intended for tablets, with a careful hand they are playable on a decent size phone screen, iPhone 5 users may have trouble with them though.
While it's far from a perfect game, Flow Free still managed to capture my attention for more hours than I would have thought. Some of the more difficult puzzles had me stumped for days at a time, and one particular puzzle took me weeks to figure out. If you've been keeping an eye open for a simple free mobile game to kill time with, then Flow Free may be exactly what you need.