Mr. Keesing was Anne Frank's teacher before she and her family went into hiding in the Annex. Mr. Keesing thought that Anne talked too much in class. He assigned a composition for her to write as punishment. This composition was to be called 'Quack, Quack, Quack, Says Mrs. Natterbeak.'
Anne decided to put a great deal of effort into creating this composition. She decided that she wanted to write it as poetry. She sought help from a friend, who had skill in writing poetry. In the end, the poem was a funny one about a family with a duck as a mother and a swan as a father. In the poem, "the baby ducklings were bitten to death by Father because they chattered too much" (The Diary of Anne Frank). This was supposed to symbolize Mr. Keesing punishing Anne for talking too much in class.
When Mr. Keesing read Anne's composition, he was amused. He was not at all offended, and saw it as a joke. After this, he let Anne talk in class without assigning her any extra work for it. He also referenced her poem on occasion.