Back when I was younger, I used to take apart the old disposable cameras I'd take to camp to have fun playing with the parts and doing a bit of exploring. One year I ended up with a camera with a flash, and got a bit of a surprise.
After having removed the cardboard I took a moment to look it over, with the thumb of one hand resting along the AA battery on the bottom. As I'm finishing my evaluation of the piece, my index finger from my other hand touched the top of the flash, which sent a current up one arm and back down the other, setting off the flash.
This caught me completely by surprise, because I'd always thought batteries only conducted if there was a complete circuit between the poles, and even holding a battery by both ends at once I've never noticed a charge before. What exactly happened here?