A method of revealing hidden content associated with a printed substrate using a handheld display device having touch-sensitive screen and a transceiver for sending and receiving digital information. The method comprises the steps of: imaging an area of the substrate containing printed content and generating image data using an optical sensor; determining interaction data using the image data, the interaction data identifying a substrate identity; retrieving display data comprising first data corresponding to the printed content and second data corresponding to the hidden content; and displaying, on the touch-sensitive screen, display information based on the display data. The display information includes first content corresponding to the printed content and second content corresponding to the hidden content, thereby revealing the hidden content on the screen.