Although newborns react primarily by reflex, babies begin making choices and actively exploring the world around them at 3 to 4 months. Babies this age are capable of bringing interesting objects to their mouths and develop preferences for toys, which they investigate by sucking on them and manipulating them with their hands. Because babies do not yet understand that some objects shouldn’t be in their mouths, adults need to keep the home environment free of potentially harmful items while still allowing the infant to explore.
Shelov, S. P. (Editor-in-Chief). (2004). Caring for Your Baby and Young Child: Birth to Age 5. The American Association of Pediatrics. Revised edition. New York: Bantam Books.