The blow sensor is not only for blowing, it will generally register any bending of the piezo disk. I posted it because if it can register a bit of air, then it can probably also register the movement of an eyebrow
when attached close to it on the skin.
when you are referring to the picture here http://www.arduino.cc/en/Tutorial/KnockSensor
without the resistor the voltages created by the piezo will be very high, since there is no path to ground. the input pin consumes virtually no current at all. all electrons will "queue up" in
front of that input pin. leaving the resistor out *might* damage the chip
(the chip has some protection built in, and when one pin is damaged there are often lots more left functioning anyway)
But if you take too low a resistor, the piezo will be sort of shorted to ground and you will not see a signal.
be sure to use a resistor of several megaohm, as the example says.
also, the piezo requires a 5 volts voltage difference
between it's two wires, that difference will not be there when one wire is connected to the 5Volts terminal but the other is ONLY connected to a high-impedance input pin. This setup without looks very much like a piezo with just one wire attached. the resistor closes the loop towards the Ground terminal of the board. only then will the voltage difference on the piezo be 5 volts. voltages are always "experienced" with reference to ground, and therefore you will need some connection to the ground terminal.
it's the same "experience" as grabbing the mains while on rubber tiles, compared to grabbing it when standing in a puddle ;D