How can you absorb another atom? You'll have to move towards the other atom, the nearer you'll be, the fastest you'll absorb it. Of course, you can absorb atoms if they are smaller than you, if not, you'll be absorbed.
Ok, so your goal is to absorb small atoms and avoid the big ones. Although at first the game seems to be always the same, each level has different goals and challenges.
If you want to try something new and different, Osmos is the game.
The demo includes 7 levels.