Nina has the habit of eating non-food substances. nina's behavior is considered abnormal because

She have a Pica disorder. Pica disorder is eating disorder, which involves eating non-nutritive things. Usually occurs in young children in first years of life and it is considered developmentally appropriate (eating sand, dirt...). However, if it last more then a month, then it is considered like a Pica disorder. This disorder can also develop in adults.