During World War II the allies had a problem, they needed a way to identify friend from foe during deployment in France to limit casualties among the Allies, so the Allies developed a technique to Identify “Friend” from “Foe” and reduce the cases of feckless injuries and needless death due to friendly fire with a device known as the “Cricket”. The Cricket made a clicking sound that made a click once to make two sounds and the ally would make two clicks, four sounds in response.
From commonly available British kids toy to war winning Military IFF device:
The device made of one or two pieces of metal made a sharp and audible clicking sound of an indeterminable origin so as not to give the GI or Tommy away, a clicking sound would make a noise from everywhere and anywhere, so would not betray the infantryman’s location using it. A lot like RADAR or ASDIC/SONAR pings that bounce back off solid objects made today. And the ally on hearing the sound would click back two clicks to confirm their location. It was a very good idea, but the tragedy was that the German soldiers cocking their rifle and loading a round, made the same sound as two clicks or four sounds too a SNAFU of war. In footage below you see of World War II you also here a call sign being exchanged, however in reality this did not happen in W.W.2 with a watchword.
Cricket in action:
The clicker aka the “Cricket” the Allied “IFF” battlefield solution.
“Cricket” picture, courtesy of Aliexpress.com (key word:clicker&cricket)