Even after exhaling as forcefully possible one still has air in the lungs?

Even after exhaling as forcefully as possible, one still has air in the lungs because the lungs are not capable of completely emptying themselves. This is due to the fact that the lungs are elastic and recoil after each breath, creating a negative pressure within the lungs that holds a certain amount of air in the alveoli. This residual volume of air is necessary to maintain the structural integrity of the lungs, prevent collapse, and ensure continuous gas exchange. The amount of residual volume varies between individuals and can be influenced by factors such as lung size, age, and lung health.