In short: At 4-6 months, the child moves from an "observer" to an "explorer": he learns to roll over, reaches for toys, and actively babbles. This is the time for sensory games and the first complementary foods (if indicated).
This material is for informational purposes only and does not constitute medical advice and is not a substitute for medical advice. If you are concerned about your child's development, consult your pediatrician or pediatric neurologist.