The team connected a chalk-holder to two strings. Each string is connected to a pulley and a weight. As the chalk moves, it pulls on the strings, causing the pulley weights--each of which is encased in a PVC tube on one side of the board--to go up and down. Using ultrasonic sensors at the bottom of each of the tubes, the String2string software can figure out how far away the weights are, and thus calculate--using some fancy geometry--the position of the chalk relative to the board. It can then duplicate that same movement on a digital canvas, recreating any writing or drawing done on the board. In theory, the team says, the technique will work with boards of any size, and would presumably translate easily to large pads of paper or whiteboards.