Why is Mrs. Johansen’s response to the German soldiers so clever ?

Once everyone has arrived, Uncle Henrik says that it is time for him to leave. He leaves through the front door. Shortly after that, a German soldier knocks and demands to know why so many people have gathered at the house. Mama explains that they are holding a vigil for her Great-Aunt Birte. The officer notices that the casket is closed, even though it is Danish custom to keep the casket open to allow mourners to say good-bye to their loved one. The soldier asks why the casket is closed, and Mama explains Great-Aunt Birte died of typhus and the doctors have warned them that the corpse could still be contagious. However, she offers to open it anyway if the officer wants to have a look. He slaps Mama and the soldiers leave. That was pretty clever. If she had just begged them not to open it, they surely would have.

Oof