Peter calls Anne "Mrs. Quack Quack" because she talks a lot.
Anne and Peter get along well at times, but often clash. They are the closest kids in age in the Secret Annexe, because Margot is older than them. They were not especially close before Peter’s family joined Anne’s in hiding, but knew about each other.
Peter has heard about an incident where Anne’s teacher called her Mrs. Quack Quack, and uses the nickname to tease Anne when she gets on his nerves.
PETER. I heard about you ... How you talked so much in class they called you Mrs. Quack Quack. How Mr. Srnitter made you write a composition ......
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