Kitchi Kitchi Coo (along with gentle up and down movements of the head)
Ga Ga Goo Goo (backed up with a soft tickle of the bellybutton)
Peak-a-boo (peaking from between the hands)
Peak-a-boo (let's see that again)
I see you (and start from the beginning)

Just in case you had any doubts regarding our understanding of babies (and theirs of us).