Sarah Fewell and Sarah Guin, health educators, explain how the Bod Pod® is used to determine body density and the ratio of lean muscle mass to fat mass. The accuracy of the device is within ± 1 percent of hydrostatic weighing (the gold standard for body composition analysis). Instead of dipping people repeatedly in a tank of water, they sit in a sealed Bod Pod® chamber where the pressure-volume is measured. The assessment takes five minutes to complete versus more than 45 minutes for hydrostatic weighing. (Photo by Jane Gervasoni, U.S. Army Public Health Command).