A fractal describes a shape composed of smaller versions of the same geometric figure. Break apart a snowflake, for example, and you find that the ice crystals that make up the snowflake are the same shape as the flake itself.

Osmos on iPhone works much along these lines, serving as a miniaturised version of the iPad original. It goes beyond that, though - the gameplay itself toys with geometry.

What appears to be cellular fusion one level turns into colliding astronomical bodies the next. Like a fractal it's a mind-bending, captivating sight.

Opposites attract

At its most basic, Osmos instructs you to expand a sphere by absorbing other smaller orbs. You accomplish this by tapping on the screen to eject mass, which results in propulsion of the sphere in the opposite direction. The more vigorous the taps, the more mass ejected and momentum gained.

Through the course of the main 27-level Odyssey mode the objective varies, even as the gameplay remains rooted in the idea of acquiring mass via absorption. Most stages involve incorporating enough mass to be the largest object on the screen, but there are twists.

Attracting bodies can influence movement like gravity, causing small orbs to enter orbit around a bulky central sphere. Other stages begin motionless, with the goal being to eject mass from your own sphere in order to affect movement in the surrounding orbs.

Random nature

These are creative embellishments on the core concept that are as inventive as they are challenging. By relying so heavily on physical interaction, Osmos regularly spirals into randomness.

No one attempt at a level is the same and that can be a source of pleasing variety or one of annoying inconsistency. Unfortunately, it tends to be the latter, particularly in later levels where the difficulty ramps up.

The emphasis placed on slow, deliberate movement also renders the game tedious at times. Impasse levels in which you must eject mass to push other spheres out of your way are the chief offender. Although interesting in concept, they're incredibly slow and boring. Repulsor and Sentient stages are far more entertaining thanks to the inclusion of active, almost aggressive spheres.

Ultimately, there's little to distinguish Osmos on iPhone and iPod touch from the original iPad release. It maintains an identical set of annoyances worth putting up with for the sake of its elegant, contemplative gameplay.

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