You don't even have to blow this compact pocket whistle--just push a button.  Emits a loud 120db whistle for as long as you push the button.  Useful in various situations such as sporting events, a locator in emergencies (earthquakes, collapsed buildings), referee whistle, traffic use for police, military training, dog handling and as a safety alarm for crime prevention.  4 inches long.  Uses 4 LR44 batteries (included).