The material culture of early childhood in the 21st century is characterized by an emphasis on biological age and related levels of cognitive and motor skill development. All types of objects, including diapers, toys, food products, and clothing, are divided into categories based on the age-appropriateness of a particular object. Descriptions of these categories commonly explain how each object functions to enable a child to attain a particular skill or reach a developmental milestone. Appeals to medical and developmental science are common. Diapers, for example, are marketed for babies of different ages: "newborn," "baby," "toddler," and "preschooler." Each of these categories is associated with specific developmental milestones, and diaper brands emphasize how their product can help with children's developing mobility skills–from rolling over, to crawling, to walking. Scholarship on childhood has shown that emphases on biological development are particular to contemporary western cultures and tends to reinforce ideas that childhood is a "natural" or "universal" experience regardless of time period or cultural context. Anthropological research with children from different cultures has shown, however, that developmental sequences vary widely as different cultural settings place particular demands on the mental and physical development of young children.