An excellent game.
Tell me, elastic collisions, with no drag or gravity effects?
I suppose this could represent a gas. But enough of nitpicking.
A simple concept (which is usually, and in this case a good thing) combined with good physics makes for an addictive game, made even more so by the grading recieved at the end.
I thought that maybe the background could be a little more than just plain white? Nothing distracting (well, not too much at first :p), maybe just a gentle pulse of colours?
Otherwise, a great game, especially for teaching hand-eye coordination aswell as spacial awareness and perception.