A fetal lung fish
Our developing bronchial trees in our lungs resemble glands when we are fetuses.
Only late in our mother’s pregnancy do the cells of our alveoli become thin enough to allow efficient gas exchange and mature enough to secrete the essential slippery, lipid based chemical known as surfactant. Surfactant will decrease the surface tension in the walls of our alveoli enough to ensure that when we take our first breath they will inflate with air - just like blowing a soap bubble.
To get an idea of how important surfactant is in allowing the alveoli to inflate. Imagine blowing a soap bubble compared to blowing a bubble made of tar. The presence of surfactant (soap) makes blowing bubbles easy because it decreases the surface tension in the bubble wall allowing it to inflate with less effort. Tar bubbles would be difficult to inflate because the surface tension in their walls will be high therefore requiring more effort to inflate.
If a baby is born prematurely (before ~37 weeks), the state of maturity of the lungs and in particular those cells that secrete surfactant will be an incredible factor in whether or not the child survives or dies from infant respiratory distress syndrome (IRDS) - the leading cause of death in preterm babies.