void GameWorld::CreateWall()
{
  // wall will be a static body placed at the origin
  b2BodyDef wall_def;
  wall_def.position.Set(0, 0);
  wall_ = world_->CreateBody(&wall_def);

  // get variables for the wall edges
  float left_wall = SCREEN_TO_WORLD(WALL_WIDTH);
  float right_wall = SCREEN_TO_WORLD(SCREEN_SIZE.width - WALL_WIDTH);
  float top = SCREEN_TO_WORLD(SCREEN_SIZE.height);
  
  // create and add two fixtures using two edge shapes
  b2EdgeShape wall_shape;
  wall_shape.Set(b2Vec2(left_wall, 0), b2Vec2(left_wall, top));
  wall_->CreateFixture(&wall_shape, 0);
  wall_shape.Set(b2Vec2(right_wall, 0), b2Vec2(right_wall, top));
  wall_->CreateFixture(&wall_shape, 0);
}