From Aesops's Fables : The Dog & The Shadow

A dog was crossing a plank bridge over a stream with a piece of meat in his mouth, when he happened to see his own reflection in the water, He thought it was another dog with a piece of meat twice as big, so he let go his own meat, and he jumped at the other dog to get the bigger piece of meat. But of course he did not get any of the meat, for one was only a shadow and the other meat was carried away by the current.