demos.stack = {};
  demos.stack.initWorld = function(world) {
          var sd = new b2BoxDef();
          var bd = new b2BodyDef();
          bd.AddShape(sd);
          sd.density = 1.0;
          sd.friction = 0.5;
          sd.extents.Set(10, 10);
  
          var i;
          for (i = 0; i < 8; i++) {
                  bd.position.Set(500/2-Math.random()*2-1, (250-5-i*22));
                  world.CreateBody(bd);
          }
          for (i = 0; i < 8; i++) {
                  bd.position.Set(500/2-100-Math.random()*5+i, (250-5-i*22));
                  world.CreateBody(bd);
          }
          for (i = 0; i < 8; i++) {
                  bd.position.Set(500/2+100+Math.random()*5-i, (250-5-i*22));
                  world.CreateBody(bd);
          }
  }
  demos.InitWorlds.push(demos.stack.initWorld);