As children my brother and I were puzzled by the parental finger tapping on the kitchen table - only years later did they admit that both having been GPO telegraphists they could in that way converse in private. Occasionally they used phonetic alphabet as well
My Dad and one of his mates were both W/Op A/Gs during the war. After a few beers at the local they would often talk in Morse Code to each other. Hours of endless amusement for all ....