Detailed Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Problem:Passage: Figure 11.31 Bronchial tubes in the lungs branch into ever-smaller structures, finally ending in alveoli. The alveoli act like tiny bubbles. The surface tension of their mucous lining aids in exhalation and can prevent inhalation if too great. Answer: tiny bubbles
Solution:
Bronchial tubes in the lungs branch into ever-smaller structures, finally ending in alveoli. the alveoli act like what?