There are several different types of touchscreen monitors. Furthermore, for most of them, only using your finger, or a special stylus will activate the technology. This is because most types of touchscreen (Capacitive, resistive, etc.) require the completion of a circuit.
So using a plastic tool or stylus would act as an insulator and not allow the electricity to move! Your finger allows the circuit to fully close, by allowing the membrane to complete the circuit. Therefore, most touchscreens are fingers only!