Which developmental milestone indicates a child's understanding that objects continue to exist even when out of sight?

The understanding that objects continue to exist even when they are not visible is called object permanence. This developmental milestone is significant because it reflects a child's cognitive ability to understand that the world exists independently of their immediate perception. Typically developing children begin to demonstrate object permanence around 8 to 12 months of age. Before this understanding, infants might act as if things that are hidden or out of sight no longer exist.

This cognitive skill is crucial as it lays the foundation for more complex thoughts and understanding of the world. It allows children to develop memory, engagement in play, and more complex relationship building with people and objects around them. Mastery of object permanence is an important indicator of cognitive development in early childhood.

The other options represent different concepts. The term "little scientist" refers to children's exploratory behavior in which they experiment and learn about their environment. "Mirror neurons" are a type of brain cell that respond when an individual performs an action or observes someone else performing that action, playing a role in social cognition and empathy rather than object permanence. "Naming explosion" describes a rapid increase in vocabulary that typically occurs in toddlers around 18 to 24 months of age, which is a separate developmental milestone distinct from the concept of object permanence.