How does a Kelvin water dropper generate charge from neutral water?

[Eternals]

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?

 

[Borek]

Everything is explained in the wiki page you linked to, hard to add something to the description given there. Molecule of water is neutral, but solution is full of ions - single drop is neutral only on average, as it can easily contain few cations or few anions more. Then when the drop is created and falls it can get additionally charged due to friction.

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