[manasecret]

For the fly question:

CO2, O2, H300, or even if they produce gold atoms somehow, the flys will always have the same mass if the jar is sealed no matter where they are. Conservation of mass. So no matter what they breathe or breathe out, the weight will be the same.

But the beating of the wings would cause greater force (momentarily anyway), which is another word for weight. So that could be an answer. But the backwards motion of the wings would cause a drop in force on the scale, so it's also a wrong answer.