. . "The Three Mountain Problem was a task developed by Jean Piaget IPA: /\u0292\u0251\u0303 pja\u0292\u025B/, a Swiss developmental psychologist. Piaget came up with a theory for developmental psychology based on cognitive development. Cognitive development, according to his theory, took place in four stages. These four stages were classified as the sensorimotor, preoperational, concrete operational and formal operational stages."@en .