A Muslim child when reading the Qur'an moves his head backwards and forwards; this is popularly supposed to be a respectful bow to the sacred words that he reads, but psychologically speaking it helps him to memorize the Qur'an by regulating the circulation and making the brain a receptive vehicle, as when filling a bottle one sometimes must shake it in order to make more room. This also may be seen when a person nods the head in accepting an idea, or shakes it when he cannot take it in.