How it works: the body-fat scale.

Step 1 When you step on the scale’s electrodes, a 500 microampere current shoots up your leg. Human skin can sense currents only stronger than 1 milliamp, which is twice as powerful as this charge.

Step 2 As the current moves throughout your lower body, it encounters resistance in tissue. Water is a much better conductor than fat, so muscle — which is about 73 percent water — provides easier passage.

Step 3 The body-fat scale detects the total time it takes for the current to complete its path. The faster it travels, the more muscle and less fat you have. With this data, it computes your body-fat percentage.

It should be noted that although some companies have developed very sophisticated body-fat scales that claim to provide accurate body fat measurements, most are not that accurate.

At present, the most accurate way to measure your body fat percentage is still hydrostatic weighting combined with a series of skin-fold measurements. [via]