Piaget argued that children mature through four distinct stages, outlined by Saul McLeod on simplypsychology.org (2009). The first occurs from birth to the age of two and is called the sensorimotor period. During this period, the main learning point is object permanence. A child learns that objects and people do not cease to exist simply because he can no longer see them. Games like peek-a-boo help teach this concept. When a child searches for an object hidden under a blanket, he demonstrates an understanding of object permanence. The second phase is preoperative and occurs from approximately 2 to 6 or 7 years of age. The main focus here is on symbolic play. This is the pretend play phase, where children play at home pretending to be mum and dad, or superheroes, or the rather outdated “cows and Indians” or “cops and robbers”. Children at this age are trying on adult roles, starting to understand the roles we play in society and experiment with them. Six to twelve years is the concrete operational period. Piaget states that this is when most learning occurs. Children learn to reason and can begin to understand things in their heads without the help of manipulatives. The ideas of conservation of number, mass, and weight begin to make sense at this stage. Children realize that the amount of liquid does not change when you pour it from a short, wide bowl into a tall, narrow glass. The final stage in Piaget's cognitive theory is the formal operational stage from age twelve through adulthood. Now children and adults are able to think abstractly and logically and can test their hypotheses to prove them