Average age: 5 to 8 weeks

"During a shopping trip, I was running down the aisles with Sam in his car seat in the cart. For the first time, he looked right at me and smiled. It seemed like he was enjoying himself, so I kept at it!"
-- Leslie Netta, Sam's mom

Though a 1-week-old may give you a gas grin, you may not see a social smile -- where your baby reacts to your facial expressions while maintaining eye contact -- until he's 2 months old. "Smile and coo at him while giving a bath, dressing, or changing him, and a true smile will come naturally," says Alice Sterling Honig, Ph.D., professor emerita of child development at Syracuse University in New York.