In nature, there tends to be one dominant rule – eat, or be eaten. This seems to apply in the microscopic world as well. In the abstract game Osmos, by Hemisphere Games, this simple mechanic is foremost among the challenges.

You control a bright blue orb, surrounded by other orbs, on a dark surface in a 2D world. You can absorb those smaller than you – which are demonstrated by their dark blue color – but those orbs larger than you will absorb some of your material instead if you touch them. The larger orbs can be identified by their reddish outline, but as you grow in size, the outline becomes thinner and paler until it also switches to blue – meaning you can absorb them without worry. The bulk of the challenges in the game consist of absorbing smaller orbs and growing in size. The more you absorb, the larger you grow. Each level has different goals, but the mechanics are the same – eat, or be eaten. 

Moving around in the game further adds to the challenge. You use the mouse to steer yourself. Pressing the mouse button will cause your orb to squirt out some of the substance you are comprised of. This has the effect of making you move in the opposite direction of your mouse clicks, but you also shrink a bit in the process. You'll continue moving in the direction you indicated, unless gravitational forces pull you in a different direction, or you run into something larger than you (which will strip away your material). So even the process of moving towards smaller orbs – or away from danger – is something that needs to be calculated into your moves. If you shrink to such a point that you cannot win, a warning will pop up at the bottom of the screen. You can then choose to restart the level from the beginning, or just steer yourself into the closest red circle and end it all. There isn't a way to save your progress in the middle of a challenge, but the individual challenges are short, so it isn't much of an irritation.

You can slow down the action on the screen, allowing yourself more time to react. You also can speed things up, but playing at the slowest speed is generally preferred. If you are having trouble with a level, it's also possible to set it up to randomly arrange the level, rather than stay with the preset design. I've found that this actually can make some of the levels easier, oddly enough. 

The graphics are polished and smooth, with the color contrast of the orbs making it easy to tell at a glance what's a threat, and what is your prey. The music consists of ambient electronica, surprisingly soothing given how tense the game can make you feel at times. The game can get a bit redundant after too long – at least when it's not getting tooth-grindingly frustrating. Fortunately, each puzzle can be completed in just a few minutes, so it's all right to play this game in small doses. Complete a level or two, then move on to another game before returning, if you'd like. It works well for a casual game. There are a total of 47 different challenges, so there is quite a bit to accomplish in the game. If played on Steam, there are 11 achievements to unlock as well, but most of them come only at the end of a chain of challenges, so they aren't pushovers to obtain.

From a moral standpoint, there really isn't too much to worry about in this game. The only sort of violence that can be found consists of absorbing the blue orbs, or being absorbed by the red orbs. Some of the challenges feature a green orb that is implied to be intelligent, but that's about the extent of it. 

Osmos is an entertaining puzzle game that can occasionally be quite frustrating. It operates on a simple mechanic that can be tough to master. For those looking for a casual game to spend just a few minutes at a time, but still provides a solid challenge, Osmos could fit the mold.