Final answer:
By 6 weeks, babies start socially smiling and making vocal sounds. Cooing is when babies make one-syllable sounds by combining consonant and vowel sounds. After cooing, babies start babbling.
Step-by-step explanation:
Developmental Milestones: Social Smile and Cooing
By 6 weeks after birth, infants typically start smiling and making vocal sounds. A social smile is when a baby smiles in response to social interactions. Cooing is a stage where infants make one-syllable sounds by combining consonant and vowel sounds. For example, 'coo' or 'ba'. After cooing, babies start babbling by repeating syllables like 'ma-ma' or 'da-da'.