Let's imagine that a flask is initially opened and in contact with the atmosphere. I am thinking that when the flask is closed with a lid, the air density inside will be kept the same as outside. As so, the pressure inside should remain Patm: P = (n/V).RT (n/V constant) However, shouldn't this be the same rationale of the straw experiment? When one, after submerging the straw outlet, closes the top with the finger and the liquid remains inside the straw? The air pressure on top is no more Patm, but less.