Jean Piaget: A Swiss psychologist known for his theory of cognitive development which includes stages such as preoperational stage where children struggle with conservation.
Egocentrism: The tendency for young children to only see things from their own perspective and have difficulty understanding others' viewpoints.
Concrete Operational Stage: The stage in Piaget's theory during which children gain the ability to think logically about concrete objects and perform mental operations but still struggle with abstract or hypothetical concepts.