The advent of initiates true social coordination in which parent and infant exchange looks and facial expressions.

Social smile

Explanation:

Newborns occasionally smile while they are sleeping. However, between the sixth and eight week of life (2 months old), babies develop a smile in response to someone else's smile, this type of smile is called "social smile" and is a reciprocal smile that invites people to interact with them.

Before that, the is no really social coordination between a baby and another person. However, the advent of social smile means that the baby's brain is developing and that he/she can get attention by smiling.

This is the true moment where parent and infant exchange looks and facial expressions.

think its caste system

explanation:

glass wants to live in a world of super heroes and villains. he have lived all his life trying to be responsible for the creating the origin story of a super hero; he schemed and caused many incidents (many of them fatal) hoping that someday someone with superpowers would show up and save the day.

he knew himself very much. he saw himself as a villain of a real-life super hero story. he was aware of his strengths (more than smart enough for being manipulative and scheming) and weaknesses (having fragile bones and having to rely on his mind to succeed).