Very crude electric shock device I made as a daft kid - 9v battery, relay, mains transformer in reverse. Wire the relay right and it switches on and off and finds it's natural resonant frequency, makes a nice racket as you zap your victim. Work out the relay circuit yourself. That was more fun than actually zapping people. (I will add I only zapped humans, definitely no animals, and the humans were all my friends and family and forewarned...)
Now I recall it was an audio transformer (from the 1970's ladybird book on building a transistor radio) that I wired the wrong way around and clicked on the terminals of a 9V battery that zapped me that led me to this more dastardly version.
I didn't even know the 555 or oscillators back then, just realised I needed to have a switch that ran like the clappers.