Medical College Professor to a girl student : "Which human body part expands 5 times its

Medical college professor to a girl student :

"which human body part expands 5 times its normal size...?"

Girl student :
"sir i can't answer this question,
It's too embarrassing...
Professor asked the same question to a male student.

Male student :
"it's the pupil of an human eye..."

Professor : "correct."
Then professor turned to the female student and said :

"listen lady,
Not only your thinking is wrong
But
Your expectations are also very high...5 times is too much"
???