A clicker is any device that makes a clicking sound, usually when deliberately activated by its user. They usually consist of a piece of thin metal or plastic 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. (source)