The grass will measure the pressure and location of your hand as it pets; additionally, it will be able to sense when someone is near it and not touching it. This data is sent to a processor that determines the character of the sound to be produced. The processor will be keeping a history of past interactions and will use this record when generating the sound.
Initially, I wanted this material to be very much like grass. I imagined flex-potentiometers as individual leaves of grass in an array that would even be sensitive to local air currents. However, I am now more interested in making the experience convenient and pleasurable for the user. Think velour. I worked in 2002 with an artist from the Media Lab who introduced me to a fabric they had developed that generated a small electrical charge proportional to the pressure applied. Weaving strips of this into another fabric, we made a rudimentary grid that functioned in the same way as 16 different MIDI drum triggers. Perhaps a similar strategy will work here...