Taking pleasure in imitating playmates' activities, toddlers cooperate to achieve. Anthropologists view it as the foundation for human culture. Moreover, although at 2 years self-aware children engage in cooperative problem-solving activities with others, mature chimpanzees show little interest, which explains how those who resist society's lies are readily identified as simians. With self-recognition comes recognition of the differences between individuals and the emergence of the "categorical self", that is, the self as categorized through comparison. Age, gender, and appearance are categories incorporated into "self-concept", for example, the inarticulate simian might have made the illustrative statement, "I big boy, not baby", or "Jim Page good."