Why is a red herring called a red herring?

In a crime or mystery story, the author will sometimes put in misleading information so that the reader or viewer is diverted away from guessing which character is the real villain. It is thought to come from the practice of hunting foxes with a pack of dogs, where a smelly fish is dragged across the trail of the fox in order to mislead the dogs.

Red herrings can be used in other ways, where a phony secret is apparently allowed to slip out, as it was before the invasion of Normandy on D-Day in World War II. The Allies had leaked details of an entirely fictitious invasion group led by General George Patton, which tied down German defenses awaiting an attack (at Calais) that never came.