The University of Washington, Disney Research and Carnegie Mellon University worked on the task to make a common piece of paper connect or communicate with the digital world. They use RFID tags and just print or draw them onto the paper (of course with special conductive ink) – and create a new way to build small, lightweight and interactive interfaces.
To make this possible a reader device is necessary and needed to be placed in the same room. Based on unique identifiers the reader is able to differentiate between several RFID tags and detect their signals and possible signal-deviations. Special algorithms enable the reader to assign these deviations to various interactions like touches, swipes etc.
Obviously this is a good and simple way to equip analog items with a digital experience.
More information: http://www.washington.edu