Jean Piaget’s notion of object permanence refers to the ability to hold an image of an object in the mind even when it is not concretely present. Piaget developed this concept while studying the behavior of his own children. He noticed that before the age of eight or nine months, if he removed an object of interest from his child (e.g., a rattle), the child would not search for it. Once it was out of sight, it was out of mind. After the development of object permanence, however, the child displayed searching behavior. For example, if Piaget removed the toy from the child and hid it behind a pillow, the child would move the pillow to find the object. This searching behavior shows that the child can think about the object even when it is not present.