Why are babies facinated by other people's mouths?

I could say "out of the mouths of babes" and that many adults have little to nothing to say that is worth the air they breath ... could that be why they are facinated! LOL Seriously, babys are much more alert that some people realize and part of it is also natural instinct that the baby sees things moving such as eyes, mouths, hands, etc. They also relate to sound. If you accidently dropped something and it make a noise I've seen babies actually jerk their little bodies. Most of us coo along and talk baby talk to the wee ones or make funny noises to get them to stop crying so part of it is a comfort zone as well. I just held a 6 week old baby last night at a birthday party (sure felt good and seems to put the whole world in perspective) and you are right ... babies are facinated with people's mouths and will go as far as pulling the person's lip or just hanging on. I haven't had a wee one around for a long time, so I was quite facinated at just how alert babies really are. Marcy