var dirx = 50;
  var diry = 50;
  var sizex = 100;
  var sizey = 100;
  var posx = 0;
  var posy = 0;

  self.resizeTo(sizex, sizey);

  function moveit() {
    if (posx + sizex >= screen.width) {
      dirx = -50;
    } else if (posx < 0) {
      dirx = 50;
    }
    if (posy + sizex >= screen.height) {
      diry = -50;
    } else if (posy < 0) {
      diry = 50;
    }

    posx += dirx;
    posy += diry;
    self.moveTo(posx, posy);
    setTimeout("moveit()", 10);
  }
  moveit();