Just like this water is a polar fluid. So we have this layer of fluid known as the alveolar fluid that is polar. Now, this, because it's polar, it basically has a high surface tension. So just like water has a high surface tension, this fluid also has a high surface tension. Now, what that means is because the fluid has a high surface tension, when we actually apply pressure, when we breathe inside these alveoli, we actually need to breathe, we need to create a high pressure to expand them, to inflate them because of the high surface tension of our fluid. And that means without any type of detergent, without any type of surfactant, which is basically a detergent inside the alveoli, we have to apply a high pressure to inflate our alveoli found within our lungs.