Nina is 8 months old and has difficulty realizing that although she can't see her favorite toy, it still exists. Her mother hides it when it is time to eat, and Nina quickly forgets that she wanted the toy to begin with. What stage of Piaget's Cognitive Development is Nina currently in, and what specific ability does she lack? Explain fully what highlights the stage and the concepts that are important in that stage. Also, fully explain why you believe what you do in your answers.