I know this may seem elementary to many on this forum so please forgive me if it does. We are having a late Christmas celebration and this question has arisen. If a fly is in a plane and flying does the plane weigh more than before the fly entered the plane? Assume the fly continues to fly, never touching anything. Also assume that the fly only displaces it's volume of air and then the plane is perfectly sealed. My hypothesis is that the plane will actually weigh less because of displaced air while the mass goes up because of increased density. As long as the fly keeps flying. I am alone and everyone else thinks the plane will weigh more. Also would the answer change if there were many many flies? Any input will be greatly appreciated.