If you ever want to test an object (a fence, an appliance, a wire, etc.) for live electrical current, lightly tap it with the back of your hand. If it's live, your arm will contract and curl away from the source, whereas if you use the palm of your hand on a live object, your muscles will contract around it and you'll be much worse off, if not dead. I learned that when I was nine or ten on a science museum field trip.
I realized yesterday that I do something similar to a woman with whom I must deal on a regular basis. I use a conventional greeting––"Hello, how are you?"––like the back of my hand: politeness expended just to assess how hazardous or volatile her mood is for the day. Sigh.