Who Started the " PEE KA BOOH " on babies?

Babies don't understand permanence when they're very young. When you walk out of the room or hide yourself behind your hands, they don't realize you still exist. This is why they'll sometimes cry when their loved ones leave the room. The look of surprise and happiness on their faces when you reappear makes this game more fun for mothers than for the babies, I think. Babies start to understand that things still exist when they can't see them after six months. Peekaboo might not be as fun for the parents after that surprise leaves their faces. :P