Three-year-old Lizzy pretend that rocks are her mother, brother and father. She will pick them up, rearrange them and pretend they are talking to each other. Lizzy is most likely in the sub-stage of Piaget's preoperational stage

Animism which is a sub-stage of preoperational stage

Explanation:

The preopperational stage is part of cognitive development that will begin around the second year to the seventh or eighth year of life.

In this example little Lizzy is pretending that the rocks have thoughts, feelings and human intent. This is called animism.

In this stage the children believes that all the things in nature has human consciousness and human purposes.