One of the major accomplishments at this stage is Object Permanence, which is the understanding that something continues to exist even if you can't see it or touch it. Object permanence represents the beginning of representational thoughts, the capacity for using mental imagery and other symbolic systems. The use of symbols and language accelerates in play and in imitation of adult behavior. Piaget described this stage largely in terms of what the child cannot do. Further, preoperational child cannot grasp the concept of conservation, which is the notion that physical properties do not change when their forms or appearances change.
Children come to understand the principles of conservation.
This marks the beginning of abstract reasoning. Teenagers understand that ideas can be compared and classified, just as object can. They apply reasoning to situations they have not experienced firsthand. They can think about future possibilities and search systematically for answers to problems. Preoperational children in particular can understand more than Piaget gave them for, and the social and cultural environment has an impact on development.