And that creates a pressure difference, also known as a pressure differential or negative pressure. And now our air molecules move from a high pressure from the outside to the inside into our lungs. Now, actually, I don't like using this term called negative pressure because it's misleading. We know from physics that pressure is simply the force that molecules exert on a certain area. Now, if we are inside a container and inside that container we have no molecules, then the absolute pressure inside that container is zero. Now, what exactly is negative pressure?