Murphy does the exact same thing when he's in the mood to play. He's the only one who does it and I think it's sooooo cute! I'll stop whatever I'm doing to watch him. I don't think it means anything, other than that he's feeling playful. 
__________________ He is your friend, your partner, your defender, your dog. You are his life, his love, his leader. He will be yours, faithful and true, to the last beat of his heart. You owe it to him to be worthy of such devotion. -- Author Unknown |