Our lungs expand and contract when we breathe. Surfactants, complex mixtures of chemicals produced by our bodies, allow lungs to inflate more easily after exhalation; they prevent the smaller air sacs from collapsing, and the larger ones from being distended during inhalation. With images like this one, which shows little bubbles made with surfactant, researchers are studying the composition of surfactant and how it functions in healthy and diseased lungs.