What will trigger a capacitance touch sensor?

[PVNRT4ME]

I've got an iPod Touch and am trying to create an electrical circuit that, when turned on, will trigger the touch sensor in a specific spot (the spot that whatever component of the circuit provides capacitance is resting on, I guess).

For the life of me, I cannot figure out how to trigger a capacitance sensor. I know what it needs; capacitance. Human fingers provide that. I've discovered that the negative side of a battery also works; but how can I control it?

 

[berkeman]

You can put an array of small metal squares over the spots, and use transistors to connect one square at a time to ground. The others will have some capacitance to ground, but much less than when the transistor is turned on.