Piaget believed that children at this stage experience two types of phenomena: pretend play and egocentrism. Pretend play is the ability to perform mental operations using symbols. Egocentrism is the inability to perceive things from a different point of view. For example, a child covers his eyes because he believes that if he cannot see someone, then they cannot see him too. When a child is seven to eleven years old, he or she is at the concrete operational stage. At this point, Piaget believed that children were capable of understanding the concept of conservation. Conservation is the principle that mass and volume remain the same despite the change in shape of objects. For example, children at this age are mentally capable of pouring liquid into different types of containers. Piaget also believed that at this age a child is capable of understanding different mathematical transformations. By age 12, children reach the formal operation stage, the final stage of Piaget's stages of cognitive development. This is the