You are playing with your baby sister and you hide her favorite toy under a pillow and she crawls to the pillow and tries to get it. by doing this, according to piaget, she has acquired:

Object permanence

Object permanence is a skill that children acquire during 0-24 months of age. Object permanence occurs in the Sensorimotor Stage of  Development (one of the four stages in Jean Piaget's theory of cognitive development). Object permanence refers to the knowledge that an object still exists, even once it is hidden. Object permanence requires the ability to form mental representations of objects. 

When your baby sister knows that her toy is still there even though you have hidden it under a pillow, she possesses object permanence.