Abstract: Apparatus having a sensor unit, preferably in the form of a sensor pen, and printed material is described where the two interact in a wide variety of ways. In particular, the printing on the printed material is configured to have different properties, where the difference is not discernible to the naked eye, for example differences in infrared absorption. A variety of arrangements are described, for example for use in early learning activities, and for testing and revision activities using multi-choice question papers. By using good design engineering, the number of different types of reaction from the printed material that the sensor unit can discriminate between may be as many as several tens, or even more than 100.