Kelvin Water Dropper

I was looking around for science demonstration that I can do for kids and found this video of Professor Walter Lewin showing an apparatus called the Kelvin water dropper.

It works by positive feedback of electrical charge separation. Once one bucket is slightly more positive than the other bucket, it will get more and more positive, building up voltage between the buckets. When the voltage is high enough, electrical charges can travel through air, making mini lightnings (sparks.)

Professor Lewin also has an interesting interview here.