About 9 Months: Selective Smile
This is the age when your baby starts to know you as special and distinct from other people, but there's a downside to that: Stranger anxiety sets in. Your formerly friendly baby is likely to stop smiling at strangers. While it can be disappointing that your baby doesn't "perform" quite as well, distinguishing faces is actually a sign of healthy development.
Also worth noting: Because your baby's sense of object permanence is stronger -- he knows that something exists even when he can't see it -- you can encourage him to smile by playing peekaboo. "Now you see it, now you don't" is the ultimate laugh-getter at 9 months.