This simple lie detector is more accurately a Galvanic Skin Response (GSR) sensor.
It is just a cut 9V LEGO motor wire and some aluminum foil wrapped around your fingers with tape.
It is popularly known as a lie detector, but is also used in Biofeedback conditioning.
The theory is that; the more relaxed you are the dryer your skin is and so the higher the skin’s electrical resistance.
When you are under stress your hand sweats and then the resistance goes down.
Oh, by the way, there is a rather ominous warning in case you decide to build this device:
Warning: You should only use this sensor with an RCX running on battery power. Operation with the RCX plugged into a AC source could result in electrocution.