A: You'll know because it will happen consistently and in response to interactions with you. The ability usually develops at about 6 weeks (before then, what looks like a smile could be gas or even a startled grimace). Seeing your baby's gummy grin for the first time is something most parents never forget -- and once your baby starts smiling, you'll want to do anything and everything to keep her doing it. Smiling back and making plenty of silly faces will encourage your baby to do the same. --Sandra Y. Lee
Originally published in Child magazine, November 2002. Updated 2009.