I have a fancy scale that in addition to showing my weight, shows me body fat percentage and body water percentage.

It works by sending a small electric current up one leg and down the other, and by some magical means purports to tell my my body fat percentage. I make no assertions about its accuracy, other than to say it is very consistent. It will give me exactly the same reading to the nearest one-half percent ten times in a row. According to the scale, my body fat percentage varies day to day from 14% to 16%.

However, there is one thing about it that makes me doubt its veracity. If I am careless and eat an extra piece of chocolate cake or engage in other evil behavior enough to run my body fat readout to 16%, if I then go out and exercise with a hard two-hour kayak trip, or maybe a four hour hike in the mountains, when I get back the body fat readout will have dropped to 15% or maybe even 14%. It doesn't seem reasonable to me that I could have lost more than two pounds of fat in those few hours, particularly since my overall weight won't have gone down that much, or possibly even increased depending on how what I ate during the exercise and how much I drank.

This behavior from the scale is absolutely consistent.

So, how do these scales measure body fat, and why does it give me these very consistent results that cannot be true?

tanstaafl.