A Kelvin dropper is a device that generates electricity by simply letting water pour down through two metal rings. While the setup is fairly complicated, the effect is incredible: you obtain a couple of thousands of volts that generate a spark between two metal balls.
Thunderf00t has created the experiment with minimal cost and it is something we all can do at home. It has to do with the fact that when the water drops in a tiny row you can get a plastic cup that was previously charged with charged particles (electrons, if you generate those via friction between the plastic cup and your hair) and change the course of the falling water.
Did this experiment a couple of times for my friends and they couldn’t believe that water, not being any magnet all all or not having any current through it could be attracted by the plastic cup.
This is science, bro!