Hang on to your own bone

Fanny the farm dog was pretty smart, but one day she got the shock of her life because no-one had ever told her about mirrors. As a special treat, she was given a big bone. She took it down to the river bank to enjoy it in peace. As she stood there with the bone in her mouth, she looked at her reflection in the water. And what did she see? Another dog with a bone in its mouth! She wanted the other bone as well as her own, so she opened her mouth to bark and her bone fell straight in and sank to the bottom.