They usually consist of a piece of thin metal held in a casing so that the metal is slightly torqued; depressing one end of the metal causes it to pop out of alignment and releasing it causes it to pop back into alignment, each time making a sharp click.

With some clickers, the user depresses the metal directly with thumb or finger; with others, a button extends above the surface of the casing so that depressing the button makes the metal click.

Use

• In contemporary society Clickers are often used by pet owners (particularly dogs and Gambian pouch rats) as a behavioral tool.

• In World War II Clickers were used by Allied paratroopers preceding and during Operation Overlord as a way of covertly identifying friend from foe. A soldier would click once and if two clicks were received in return from an unidentifiable soldier then his identification was confirmed.

• Clickers are also used as a handheld counting device, sometimes digital but more commonly mechanical, used to keep a count of the numbers of people entering a venue. It is often used by nightclub doorstaff to make sure fire limits are not exceeded.

Other meanings

• A Clicker is the name of the tool used by a weather presenter to change the map - the clicker can either be remote, or attached by a wire.

• The Clicker is a term sometimes used to refer to a television remote control, as early ultrasonic remote controls were similar to clickers.

• A clicker is also a device used in archery which tells the shooter when to release the arrow. It consists of a short length of metal (or carbon) screwed into the riser of the bow. The arrow is placed in between the clicker and the pressure button on the bow. When the arrow is pulled through the clicker, the clicker hits the clicker plate with a strong, positive click telling the archer to release. Misadjusted clickers cause a weak buzzing sound.

• A clicker is also a term used to describe a player in an MMORPG, like World of Warcraft, who plays only with the mouse rather than binding actions to a key. This is often used as a negative term, as clicking buttons tend to be less efficient than binding, giving you a slower reaction time.

• A clicker is a derogatory term humans use to refer to humanoids in the 1962 science fiction movie The Creation of the Humanoids, and to robots in Alan Moore's Top 10 comics.

• A clicker is a training behavioral device used as part of Classroom Performance Systems or Audience Response System technology. According to Kaleta and Joosten (2007), "Student Response Systems (SRS) or clicker systems are increasingly being adopted by faculty and are transforming traditionally passive lecture courses into stimulating and active classes...Once in the classroom, faculty can pose questions at pedagogically strategic moments in the course and allow students to respond using the second component of a Student Response System, the personal response unit or clicker, to send their responses to the classroom computer" (p.2).