Radio in the Bathtub

Obligatory reference category: When discussing GFI, always refer to the classic example of why you need GFI.

How does an Electrical Outlet Work?

For instance, if a radio fell into the bathtub while you were bathing, you would be in for a bit of trouble to say the least. With a standard two or three prong receptacle, the power would continue to pass through the water and you. In all likelihood, you would be electrocuted. If, however, that radio was plugged into a GFI receptacle, the receptacle would sense the lack of electricity returning to pass through the neutral (since it was passing through you instead) and trip, thus killing the power to the radio and potentially saving your life.

website: http://www.wisegeek.com/how-does-an-electrical-outlet-work.htm