A dog, carrying a piece of meat in his mouth, was on his way home. To cross a running brook, he had to walk over a narrow plank. As he did so, he looked down and saw his own shadow reflected in the water below.
Mistaking it for another dog with another piece of meat, he decided to take that one too. He snapped at the shadow in the water. But as he opened his mouth, his own piece of meat fell out, dropped into the brook, and was lost forever.
Moral: Beware lest you lose the substance by grasping at the shadow.