I have been reading the threads about using car batteries and household current. Basically both can be dangerous. But here is a little device that I built as a kid and it will develop a shock from a mild tingle to a pretty good sting, almost as much an electric fence. The output current is very low so it shouldn't be enough to kill (I got this circuit out of a child's electronic kit) but as always use common sense.
Pardon the crude drawing it has been years since I have done any ASCII art. If you cant read it email me and I can send you a better drawing.
Note: Drawing was plagued with spacing problems. Changed to preformatted text, and replaced every space with a nonbreaking space, so that the code will transfer well as long as the end of lines are kept intact. I also drew up a graphical schematic for those of you who have problems understanding the original ASCII drawing.
Theory of operation:
Experiment with the value of the capacitor, low values will cause a smaller shock but at a faster rate. Larger caps will cause a larger shock but at a slower rate due to the longer time needed to charge. If you want to get real fancy you could use a rotary switch to chose between several cap values. If you get a small DIP type relay you should also be able to fit this in to a small tube to be used as a cattle prod.
Note: I found my initial trial value of 4.7uF to be too small for me to even feel. Moving up to a 47uF capacitor finally let me feel the zap, when my palms were wet enough. I suggest playing around with it on yourself to see how conductive the subject needs to be, and adjust accordingly by wiping the area to be used on with a damp rag.
Final Note: A similar shock can be achieved with the static electric sparker from a gas grill. The red pushbutton kind works best, as opposed to the more "modern" grey rocker switch. Keep the lead on it, and approach someone from behind, touch them with the lead, and hit the button. Watch them jump into the air. This switch will sometimes shock you through the wire embedded along the side, but you should be okay with it as you know what it feels like because you tested it on yourself first... RIGHT?