Take the five by five square. Every letter is a row and a column, so every letter is two sets of taps: tap tap, pause, tap tap tap. Nothing to write, nothing to hide, and it goes through a wall.
American prisoners of war in Vietnam used it for years, taught it to new arrivals through the walls, and held whole conversations with it. C and K share a cell, so C is usually tapped as K.
The code that came through the wall
American prisoners were kept apart and forbidden to speak. One of them remembered a tapping code, and taught it through the wall to the man in the next cell, who taught the next. Whole conversations, jokes, names of new arrivals and news from home moved through the walls of that prison for years, one knock at a time. The men who survived it say the tapping is what kept them from being alone.
Print this page and you get a worksheet: how the cipher works, where it came from, and the puzzles to crack, with the machine and the menus left off.
Print this page as a worksheet Its sheet in the pack Try the challengesLonger, harder and written for grown-ups. Worth a look when this page runs out.