Based on my dusty memories of Piaget in developmental psychology, I'd guess the baby is interested by the fact that you can start with one thing and end up with two. It's the kind of amazing fact about the universe that we take for granted as adults, but babies have to learn it. Sort of like how babies have to learn that when an object is not in sight, it still exists.
Are you fucking retarded? Clearly you are if you expect a baby to be on the same intellect level as you- fucking retard! Have you ever had a baby? They are just discovering the world and all the sounds and smells, if this is funny to him what the fuck is your problem asshole? This is a great video of a daddy having fun and laughing with his gorgeous happy child- if anyone sees a problem with that you are fucking retarded and you have serious mental issues!
http://www.youtube.com/watch?v=cXXm696Ub…