Larana is right they use a very minute electircal current to measure resistance, fat and muscle have difference levels of resistance bone density should not change significantly your hydration level can alter the result so it is best to make sure you are not dehydrated when you use them. LOL on a scam I thought so too and researched it before I got one. I got mine from big w they have a big range i think mine were about $60, they were the weight watchers ones