Mass Attack is a simple game sitting neatly in the casual games genre - games that anyone can just dip into for a quick 10 minutes of mental refreshment.
It's beautifully simple - you just click (and hold) on the other side of the scales to produce weights (balls) that balance out with those on the other side. As long as you can get them (you get 3 chances) within 3 units of each other (i.e virtually balanced), you can proceed.
Fun.
