Goodbye Mousie by Robie H. Harris

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One morning a boy finds that his pet, Mousie, won't wake up. The truth is Mousie has died. At first the boy doesn't believe it. He gets very mad at Mousie for dying, and then he feels very sad. But talking about Mousie, burying Mousie in a special box, and saying good-bye helps this boy begin to feel better about the loss of his beloved pet.