根据Bergin(2011)的说法，皮亚杰认为，如果我们想要了解儿童的认知发展，我们不仅可以观察他们的行为，我们还必须调查孩子为什么会这样想。然后，皮亚杰相信同化过程也包括学习。“同化”指的是孩子们在感知、思想或形象等现有计划中获得经验的过程。例如，当老师问学生什么是“孤立”的时候，学生们会根据他们对“冰”这个词的认识来回答。在此之后，住宿的过程恰好适应了新的体验，老师将帮助孩子们适应他们的孤立计划。Santrock(2011)指出，认知发展分为感觉运动阶段、前操作阶段、具体操作阶段和正式运作阶段四个阶段。第一阶段是感觉运动阶段，婴儿在两岁左右开始通过抓取、吮吸和触摸来观察世界。婴儿们会学习一些象征意义的思考，例如，他们应该知道像妈妈和苹果这样的词语代表着事物。他们也可以模仿他们过去观察到的动作，比如把玩具放在篮子里或者给洋娃娃吃。然后，2到7岁的前运算阶段是孩子们开始使用更多的单词和图像来增加他们的象征性思维。在这个阶段，他们还没有逻辑思维。第三阶段是具体操作阶段，年龄为7 t0 11。在这个阶段，孩子们可以在逻辑上思考，收集和分类不同的东西，如邮票或大理石。最后一个阶段是12岁及以上的正式运作阶段。在这个阶段，孩子们可以思考更抽象和逻辑的方式。他们会考虑理想的朋友，然后开始比较。他们也可以更系统地解决他们的问题，并思考为什么会发生一些事情。
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.