Red herring is a metaphor for a diversion or distraction from an original objective. An example can be found in academic examinations, particularly in mathematics and physical sciences. In some questions, information may be provided which is not necessary to solve the given problem. The presence of extraneous data often causes those taking the exam to spend too much time on the question, reducing the time given to other problems and potentially lowering the resulting score. Red herrings are frequently used in literature and cinema mysteries, where a character is presented to make the reader/viewer believe he/she is the perpetrator, when in reality it is someone far less suspect.
The phrase may relate to saving a hunted fox by dragging a smoked herring across its trail. This act would create a diversion through the strong smell of kippers. The Oxford English Dictionary records its first written use occurring in 1686 in this context: "To draw a red herring across the track". Michael Quinion says it is unlikely that any such act ever occurred.
The use of a red herring in this way is described by James Rodwell in 1863 book against the rat. Here he describes the red herring as a "kind of witch", which when dragged along the floor to make a scented trail, is a method "commonly used for charming rats" and removing them from certain places (pp 194).