I am wondering how a Kelvin water dropper works and can generate sparks. Basically, water drips onto two sides, and the apparatus attracts positive charge to one side and negative charge to another side. The charge eventually builds up high enough to make a spark. You can see more information about it here: http://en.wikipedia.org/wiki/Kelvin_water_dropper I can understand that general idea, but what I don't understand is where the positive and negative charge comes from. Isn't a single water molecule neutral? It would have (1 + 1 + 8 = 10) protons and also 10 electrons, so the net charge of a single molecule would be 0... So what would be making the buildup of charge?