A hole in your lung
Put your fist in a slightly inflated balloon.
Your fist is your lung. The balloon represents the lung lining both inner and outer. The space inside is where a small amount of fluid lives. When the outer balloon is punctured by something (broken rib, ect), air pushes down onto your lung, and your lung 'drops'. We place a little tube into that space to re-inflate your lung, allowing healing.