Teaching your dog to sit.
Pat your dog's head and then quickly slide down it's chest. Then when you see the little light bulb in the top right corner on the touch screen, press that and then say sit down. It will take a few tries but it's worth it because after that, you can walk it and lots more! You can even feed it.
We have more cheats and tips for this game here, Nintendogs: Chihuahua and Friends cheats
Comments for this hint