Have you ever played with oil and water in a bottle? The oil sits on top of the water until you turn it over and get a lot of little droplets that float up and combine with each other again. That's the basic mechanic behind Liquid 2, a physics puzzle by OneGoodGame.
The controls are simple: use the [A] and [D] keys or the [left] and [right] arrows to rotate the playing field and get as many droplets of liquid into the goal area as possible. 20 drops is the minimum to pass each level, but you earn more stars for getting more liquid to the finish. Later levels introduce obstacles that evaporate any droplets that touch them, keys that unlock the exit, arrows that redirect the liquid, and inverted gravity. The walls in World 3 are even invisible — until the liquid touches it, or you trigger a paint geyser that exposes the nearby barriers.
The game isn't perfect; in particular, the camera focuses on the largest concentration of droplets, which can make getting that last little drop nearly impossible if your liquid's been split up a lot. It would be nice to be able to control the camera, or at least zoom out to view the whole playing area. Overall, though, there are just enough levels to be challenging and fun without becoming boring.
Just be careful pouring salad dressing after you play this.