A row of glass bottles holds layers of colored water, all mixed up. Water Sort Puzzle asks you to pour the layers around until every bottle contains just one color.
Click one bottle, then click the bottle you want to pour into. It works with a mouse or a finger.
The top color has to match the color you are pouring onto, or the target bottle has to be empty, and there has to be room left in it.
Not at first. The early levels double as a tutorial, and the difficulty grows as more bottles and colors arrive. It is more about patience than speed: look at the whole row, plan two or three pours ahead, and restart if you have trapped yourself.
Sudoku Classic makes a good partner for the same kind of patient thinking.