There are a bunch of different ways to do this. For example, if you feed Google(r) this image search,http://www.google.com/search?hl=en&tbm=isch&q=skin+resistance+circuit then you'll see a bunch of different pictures of circuit diagrams. I claim that what most of these circuits have in common is a pair of bare metal "probes", wires, metal studs, or something like this. Electrically the probes look like an open circuit (ideally infinite resistance) when nothing is touching the probes. When skin is touching the probes, a resistance, in the range of roughly 100 K ohm to 10 M ohm, is sensed between the probes. So that's most of the model right there: the circuit sees (senses, detects) skin touching the probes, as a resistor. The exact value of this resistance depends mostly on how moist the skin is. Supposedly it is possible to use a skin resistance measurement like this to detect the presence of small amounts of sweating, caused by psychological stress, e.g. that associated with lying. So sometimes circuits like this are sold, or presented as, a "lie detector" circuit. So "lie detector" is another phrase you can use for searching for searching for these kinds of circuits. Some other phrases used to describe the same thing: "skin resistance", "skin conductance", "galvanic skin response"http://en.wikipedia.org/wiki/Skin_conductanceAnother important feature these circuits all share, is that they are all powered by batteries, so that the actual voltage seen at the probes is LOW, usually less 10 volts or so, DC. A skin-resistance detecting circuit connected to the mains, 110 or 220 volts AC, would be a very bad idea, because you don't want people touching wires that are are touching wires that connected to mains voltages. So whatever you build, design it to run on batteries for safety reasons. I am not sure which circuit to recommend, because I am not sure what you like building with. For example, I have had good luck with op-amps in the past, so I would probably choose a circuit that uses op-amps. Also not sure how simple or complicated you want it, or what kind of output, sound or light, or what. Probably the most simple thing to try is to just get a digital multimeter,http://en.wikipedia.org/wiki/Multimeter and set it to measure resistance, probably on its highest scale (on mine this is 2000 K ohm = 2 M ohm full scale). Then try to measure the resistance through your hands holding the probes. When I tried this, the resistance was initially out of range (higher than 2 M ohm), but I was able to get a reading after putting some water on my hands, and that brought the resistance down to around 500 K ohm.