Q: In this task, you are given a paragraph, and your task is to construct a question from the given passage. Construct question in such a way that (i) it is well explained in the paragraph, (ii) the words may/may not be present in the paragraph, (iii) it should start with what/how/describe. Avoid creating a question that (i) is just relevant to one or two sentences, and (ii) not relevant to passage. 
You breathe in. Air goes through all the little branches in your lungs to fill up the air sacs, or alveoli. Oxygen goes through the walls of the alveoli and into the capillaries. The oxygen goes into the bloodstream. Oxygen is left and carbon dioxide is picked up. You breathe it out through your lungs. 
A:
How do the lungs work?