Expands up to six times its size

Mr. Baldwin, the biology teacher called on Mary, "Can you tell me the part of the body that, under the right conditions, expands to six times it's normal size, and state the conditions." Mary gasped and said in a huff, "Why, Mr. Baldwin! That is an inappropriate question and my parents are going to hear of it when I get home!" She sat down, red-faced. "Susan, can you tell me the answer?" asked Mr. Baldwin. "The pupil of the eye, under dark conditions," said Susan. "Correct. Now Mary, I have three things to say to you. First, you have not studied your lesson. Second, you have a dirty mind. And third, boy are you going to be disappointed someday!"