Cognitive Milestones

Birth to 3 months: "Focus on moving objects, including the faces of caregivers (verywellmind.com)", means that at this age a child is learning to focus on one thing at a time

3-6 months: "Recognize and react to familiar sounds (verywellmind.com)", this means that while the child may startle at unfamiliar sounds, the ones that they are around on a daily basis will not bother them and they will start to respond to their caregivers voice.

6-9 months: "Understand the differences between animate and inanimate objects (verywellmind.com)." they are beginning to understand the concept of staying still and moving

9 months to a year: "Understand the concept of object permanence, the idea that an object continues to exist even though it cannot be seen (verywellmind.com)". where children before would probably cry and scream when their caregiver left their line of sight they now understand that just because they cannot be seen does not mean that they are not there.