Lick'O'meter is the most basic type of resistive touch switch, in its simplest form. 
The operation is simple. 
the transistor is used as a switch.  If one of the electrodes is touched, nothing will happen.
But if both electrodes are touched together, then a small amount of current will flow through our skin to the Gate of the transistor. 
The transistor will then switch from cut-off to saturation mode,
A connection Between Drain and Source is created and current will flow to the LED.
The resistor keeps the Gate pulled down to ground when no electrode is touched, so it does not float. 

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