Electronic – How the pin2 of 555 timer triggers this simple circuit

First of all, I will go through my concept which make the doubt.

When we connect positive side of LED to a positive terminal of a battery and negative of LED to floor (not to -ve of battery), current will not flow, as -ve of battery is not at a potential equal to that of floor (0V). But if I hold by standing on floor of phase line of commercial electric circuit, I get shock because neutral line is at same potential as ground. (Correct me if I'm wrong.)
Now look to this circuit:

_{(source: circuitsgallery.com)}

Figure above shows a touch sensor using 555 timer. It work only if a low pulse applied to pin 2. But how when we touch the plate it get low pulse? Although we consider -ve terminal to be 0v, in practice, I think it is not. As like my first example, I expect the current never flow and pin 2 stays at high voltage. But it is not so.

Then what is my mistake?
Pls help me I am a beginner.
Sorry for my English.

EDIT

Some answers tell me "how the 555 works" or "how sensitivity of circuit increase". But my question is different from that. May be this is due to unclear data from my question. My question is how the pin 2 trigger (or the voltage at pin 2 become less than \$\frac{1}{3}V_{cc}\$ ) when I touch the plate?