Looking at this Clock-a-doodle-doo is just like looking on a child playing with their own magnetic particle based white board.
clock-a-doodle-doo using its mechanical pen to pull the magnetic particle from the back layer to the from to show you the time. And on every minute changes, a full bar of block behind the display will ‘erase’ the drawing by pulling it back to the original state.
See Clock-a-doodle-doo clock in action:
“This unique electromechanical clock is like a cross between a Magna Doodle and an Etch-a-Sketch.”
— by TechnaBob
( Source: Technabob )