Humans uniquely use systems of symbolic communication such as language and art to express themselves and exchange ideas, as well as to organize themselves into purposeful groups. Humans create complex social structures composed of many cooperating and competing groups, from families and kinship networks to political states. Social interactions between humans have established an extremely wide variety of values, social norms, and rituals, which together undergird human society. Curiosity and the human desire to understand and influence the environment and to explain and manipulate phenomena have motivated humanity's development of science, philosophy, mythology, religion, and other fields of knowledge.