Before you start rubbing your dog’s belly, look at his posture. If his body looks fluid and loose, then he’s feeling pretty relaxed and happy. If he looks tense, then he may not be very receptive to having his belly rubbed. If your dog is sleeping, let him sleep rather than waking him up to rub his tummy. Approach your dog. If your dog rolls onto his back as soon as you get closer to him, he is likely demonstrating submissive behavior. This submissive behavior may be accompanied by other submissive actions, such as licking the lips and tucking the tail. To the untrained eye, these actions look like an open invitation by your dog to rub his belly, but this is not necessarily the case.  If your dog becomes submissive when you approach him, he may actually be intimidated by you and will not want you to rub his tummy right away. To make him more feel comfortable with you, get on the ground a small distance away from him. Call out to him and let him come over to you. When you call him, do not reach out to get him to come closer. Your dog may present his belly as a form of submission, but he may also be showing that he trusts you or wants to play. It is not always obvious what type of behavior your dog is showing. If you are unsure if your dog is being submissive versus being trusting or playful, your veterinarian can help you more fully understand the differences. When a dog lies on his back, he is putting himself in a vulnerable position. Provided that you and your dog have developed a close bond, he will probably be comfortable with allowing you to rub his tummy.

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Write a summary of the previous text in Vietnamese:
Quan sát điệu bộ của chó. Xác định khi chó tỏ ra biết nghe lời. Hiểu lý do chó phơi bụng.