What makes a baby smile?

Recently, this week actually, we have discovered that Will has started to do "social smiling" (that's what the doctor called it), meaning he smiles back at you when you smile at him, not just randomly. Of course this is great fun for us, and even better than that, my sister discovered a noise that actually makes him smile and even give a little baby laugh a little once in a while. It is great. He has started to "talk" much more lately as well. He is a very alert and active baby, with quite a bit of wake time throughout the day. Please excuse the poor video making--I was holding the camera myself with him on my lap, trying not to block my mouth so he could see it. There is a good little "laugh" about half-way through, so it's worth toughing out. : )