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Why do you think Janie washes and irons the polka-dot dress, hangs it in her closet, and touches it every day?
The dress reminds Janie that she has a real past. With all the confusion in her life, Janie latches on to the dress as something that makes sense and brings comfort. Perhaps Janie also feels that, aside from the milk carton, the dress is the only physical evidence that proves her Jennie Spring theory.