Children learn that they are separate from the environment. They can think about aspects of the environment, even though these may be outside the reach of the child's senses. In this stage, according to Piaget, the development of object permanence is one of the most important accomplishments.Object permanence is a child’s understanding that objects continue to exist even though he or she cannot see or hear them.Peek-a-boo is a good test for that. By the end of the sensorimotor period, children develop a permanent sense of self and object.