根據Bergin(2011)的說法，皮亞杰認為，如果我們想要了解兒童的認知發展，我們不僅可以觀察他們的行為，我們還必須調查孩子為什么會這樣想。然后，皮亞杰相信同化過程也包括學習。“同化”指的是孩子們在感知、思想或形象等現有計劃中獲得經驗的過程。例如，當老師問學生什么是“孤立”的時候，學生們會根據他們對“冰”這個詞的認識來回答。在此之后，住宿的過程恰好適應了新的體驗，老師將幫助孩子們適應他們的孤立計劃。Santrock(2011)指出，認知發展分為感覺運動階段、前操作階段、具體操作階段和正式運作階段四個階段。第一階段是感覺運動階段，嬰兒在兩歲左右開始通過抓取、吮吸和觸摸來觀察世界。嬰兒們會學習一些象征意義的思考，例如，他們應該知道像媽媽和蘋果這樣的詞語代表著事物。他們也可以模仿他們過去觀察到的動作，比如把玩具放在籃子里或者給洋娃娃吃。然后，2到7歲的前運算階段是孩子們開始使用更多的單詞和圖像來增加他們的象征性思維。在這個階段，他們還沒有邏輯思維。第三階段是具體操作階段，年齡為7 t0 11。在這個階段，孩子們可以在邏輯上思考，收集和分類不同的東西，如郵票或大理石。最后一個階段是12歲及以上的正式運作階段。在這個階段，孩子們可以思考更抽象和邏輯的方式。他們會考慮理想的朋友，然后開始比較。他們也可以更系統地解決他們的問題，并思考為什么會發生一些事情。
According to Bergin (2011), Piaget believed if we want to understand children's cognitive development, we cannot only observe what they do but we must investigate why children do and think like that. Then, Piaget believed that assimilation process also including in learning. Assimilation means the process when children get experience into existing schemes such as perception, thought or image. For example, when a teacher asks her students what 'isolated' means and students will answer it based on their knowledge of the word ice. After that, the process of accommodation happens to adapt new experience and the teacher will help the children to accommodate their scheme of isolated.Santrock (2011) pointed out, the cognitive development has four stages such as sensorimotor stage, preoperational stage, concrete operational stage and formal operational stage. The first stage is Sensorimotor stage when the infants at age around two begin to observe the world through grasping, sucking and touching. The infants will learn the symbolic thought for examples they should know the words like mama and apple are represent the things. They also can imitate actions that they observed in past such as putting their toys in basket or giving the dolls food. Then the preoperational stage about ages 2 to 7 is when the children start to use more words and images that increase their symbolic thinking. In this stage they cannot think logically yet. The third stage is the concrete operational stage about ages 7 t0 11. In this stage, the children can think logically, collecting and classifying things into different sets like stamps or marbles. The last stage is the formal operational stage at age 12 and older. In this stage, the children can think more abstract and logical ways. They will think about the ideal friends are like and start to compare things. They also can solve their problems with more systematic and think why something is happening.