Agreed it is a poorly worded question with no clear definition of what “weight” is, so open to different interpretations.
Perhaps asking where the fat molecules go would be a better question.
Even so, as someone with limited biological knowledge, it came as a surprise to me.
It’s not something I’ve ever thought about all that much before, I suppose I just assumed that as fat people lost weight, they would have very fatty poo.
I wouldn’t have immediately guessed that the individual hydrogen, carbon and oxygen atoms in solid fat got recombined in to gas, even though I already knew that trees make wood out of gas.