Why is it that some electrical appliances (especially high wattage ones) use a 3-pin plug instead of a 2-pin plug? I know that the 3rd pin is the ground pin but I don't know why do we need to ground some appliances and not others? Also, why is it that for a 2-pin plug, we sometimes have one pin slightly wider than the other? I am guessing this has something to do with the polarity of the voltage, right? If so then which pin corresponds to which polarity?