Educational television has no commercials and has programs for children that many parents approve of the most famous of these is Sesame Street,which tries to give preschool children a head start in learning the alphabet(字母) and numbers. It also flies to teach children useful things about the world in which they live.Even though most parents and educators give Sesame Street and shows like it high marks for quality, some critics argue that all television,whether educational or not,is harmful to children. These critics feel that the habit of watching hours of television every day turns children into bored and passive(被动的) consumers of their world rather than encouraging them to become active explorers of it.