I was doing some research about measuring body composition. I learned that being fat may interfere with our body to conduct current:
"Bioelectrical Impedence
Description: By standing barefoot on metal foot plates, an undetectably low voltage electric current is sent up one leg and down the other. Since fat is a very poor conductor of electricity, a lot of fat will impede the current more so than a lot of lean tissue. By measuring the resistance to the current, the machine estimates the percent body fat."