Bathroom scales that register fat% and water%?

HOW exactly do they do this? how can the scale tell how much water or fat you have in your body???

Public Comments

1. Runs a small electrical signal through your body. Fat, muscle, and bone all react to this differently. The scale calculates body fat based on that. Its not dangerous, but not terribly accurate either.
2. It measures impedience of a minute electrical current being passed from one foot through your body to the other foot. The current travels at different speeds through different types of tissue and can give you approximations as to body fat % and whatnot but *only* if you're already programmed in your height, sex, etc. to allow it do the additional calculations requred.