anchor.diff:  
   == Exercise
   
  -Can you “invert the fire effect into a small waterfall? Here some tips:
  +Can you "`invert`" the fire effect into a small waterfall? Here some tips:
   
   *  Change the Red and Yellow color to Cyan and Blue
   *  Invert the velocity vector (direction) by using a negative number
  
positive.code:   @Override
    public void simpleInitApp() {
  
      ParticleEmitter fire = 
              new ParticleEmitter("Emitter", ParticleMesh.Type.Triangle, 30);
      Material mat_red = new Material(assetManager, 
              "Common/MatDefs/Misc/Particle.j3md");
      mat_red.setTexture("Texture", assetManager.loadTexture(
              "Effects/Explosion/flame.png"));
      fire.setMaterial(mat_red);
      fire.setImagesX(2); 
      fire.setImagesY(2); // 2x2 texture animation
      fire.setEndColor(  new ColorRGBA(1f, 0f, 0f, 1f));   // red
      fire.setStartColor(new ColorRGBA(1f, 1f, 0f, 0.5f)); // yellow
  
negative.code: [source,java]
  ----
  
      ParticleEmitter fire =
              new ParticleEmitter("Emitter", ParticleMesh.Type.Triangle, 30);
      Material mat_red = new Material(assetManager,
              "Common/MatDefs/Misc/Particle.j3md");
      mat_red.setTexture("Texture", assetManager.loadTexture(
              "Effects/Explosion/flame.png"));
      fire.setMaterial(mat_red);
      fire.setImagesX(2);
      fire.setImagesY(2); // 2x2 texture animation
      fire.setEndColor(  new ColorRGBA(1f, 0f, 0f, 1f));   // red
      fire.setStartColor(new ColorRGBA(1f, 1f, 0f, 0.5f)); // yellow
  
  ----
  
  [.right]
  