I was having a discussion with a friend and we could not come to an agreement on the following: If you were to weigh yourself on a very sensitive set of scales, would you register the amount of air that you breathe in as an increase in measured weight? The way I see it is, you can make one of two assumptions. If you assume your body's external volume doesn't increase at all when inhaling, and lung volume increases due to the compression of your diaphragm, then mass is being added to the system and should register on the scales. If your external volume increases by the same amount that your lung volume increases, the increased buoyant force on your body cancels the increased mass in your lungs and no change in the measured weight will be observed. I assume that the inhalation process is some combination of the two, where the measured increase in weight will be an underestimate of the true weight of the air inhaled, since the body's volume increases by some amount less than the increase of the lungs' volume. Is this correct? He was arguing that there will be no measurable change in the weight measured.