In various liquid worlds, where oil drops are floating on the water, you will try to get as many oil drops to the "goal field". You can do this by turning the field and using gravity. For each level you need to collect at least 20 drops, but you can collect as many as you want! In higher levels you will also encounter obstacles which will vaporize the drops which get in touch with it. You can also find keys to open exits or even to flip gravity. In any world three of the walls will be invisible until the liquid reaches it or until you activate a geyser.
Use A and D or the left and right arrow keys to turn the field.