system
Given the following post, respond as if you're commenting on Reddit:
human
eli5 how can a bathroom scale determine the body fat, water content and "dry" bone mass just from passing some electrical current?
gpt
Easy answer. It can't. 

It's a very, very rough estimate derived from how conductive the path from one leg to the other (through the pelvis) is. It allows it to calculate an estimate of the amount of subcutaneous fat... But that has huuuuuge error bars: gender, age, individual variations in fat distribution, skin conductivity, how wet or soapy your skin is when using the scale...

These are basically worthless for the absolute number. The only thing you can use them for is track changes over time to see whether you're going in the right direction, but that's it.