Good game play, bad scoring system
The game play works. I can change the gravity and move them into their correct spots pretty easily.
The huge problem with the game, however, is the scoring system. The player has no idea what to do to get A, B, etc. I retried one level four times trying to get an A grade for it, but I couldn't get it. I tried speed, line accuracy, and fewest number of clicks. Neither of them gave me an A for the level (I kept receiving B's). That's when I gave up and closed the browser window--not a good thing to happen for you as a game developer.
Other than that, the music is perfect. I like it. It fits right in and immerses you into the feel of the game. The sounds also seem naturally placed. The art is also great.
Anyway, keep up the good work. The game is nice but flawed in its ability to reward the player for playing it well. I suggest informing us about how we're supposed to reach the best grade and best score. Right now, the score and grading system seem extremely arbitrary.