Jean Piaget, a Swiss psychologist, has made a systematic study of cognitive development in his theory that is categorized in four stages. He observed his children and their process of making sense of the world around them and developed a model of how the mind processes new information encountered. Key Points The concrete operational stage is the third stage in Piaget's theory of Cognitive Development. In this stage, children gain the abilities of conservation of number, area, volume, and orientation. Conservation is the understanding that something stays the same in quantity even when its appearance changes Children show attainment of the concept of reversibility, seriation, transitivity as a cognitive capacity. Reversibility is the understanding that a child develops to know that things that have been changed can be returned to their original state. Children can conserve numbers (age 6), mass (age 7), and weight (age 9). Important Points The Four Stages of Cognitive Development: [ alt="5f759bb7d1079b9d815b6331 16387684950011" src="//storage.googleapis.com/tb-img/production/21/12/5f759bb7d1079b9d815b6331_16387684950011.png" style="height: 219px; width: 430px;"> Hence, it could be concluded that Rahul has acquired the Concept of conservation as per Piaget's Theory.