Air is a fluid and so is water in physical terms. Air can be a liquid if the pressure is increased. Actually the term liquid or gas is just a convenient but sometimes inaccurate way to classify fluids. An example: if we measure the flow of a liquid at relative low pressures we don’t consider the compressibility of that fluid. But on the other hand, at similar pressures me must consider the compressibility of the gaseous fluids to be accurate.

The jar with the fly inside will only register the fly’s weight when the fly attaches itself to the jar otherwise the air modules support the fly. I’m not considering the heat rise of the air inside the jar due to the poor little fly beating its wings before it lands on the jar.

Think of yourself standing inside a bucket and the bucket the is on a scale. Look at the scale then step out of the bucket. I’m sure the bucket will way less when you are no longer standing in it. Now step back into the bucket, start flapping your wings until you are no longer resting on the bucket. The scale will no longer register your weight.

Miles.