Ten-year-old Bill is playing with his baby sister, Lucy. He takes Lucy's teddy bear and hides it behind a pillow while Lucy watches. Lucy quickly finds the bear. Then Bill puts the teddy bear in a bag, puts the bag behind a chair (where he dumps out the bear), and brings out the empty bag. Lucy looks inside the bag but never tries to look for the bear behind the chair. Which one of Piaget's sensory motor substages is Lucy in