The question is somewhat badly phrased. I think many people interpret it as "Why do I lose weight?" or even "Where does that fat go when I lose weight" and ans

The question is somewhat badly phrased. I think many people interpret it as "Why do I lose weight?" or even "Where does that fat go when I lose weight" and answer correctly: your body uses up your fat stores as fuel when there's no easy sugar available. Calling that a wrong answer is a bit unfair, there's nothing technically incorrect about it. However, that's not what was asked - the question is where does the mass go? After all, if you light a fire inside a box, the box doesn't get lighter. The trap is that there are by-products from the chemical reactions which have to go somewhere, and that somewhere is typically through respiration (and excretion).