There's A Huge Market Around 'Baby Sign Language,' But Little Research On Its Effectiveness
This article describes a growing method of "baby sign language", where parents teach their babies signs for simple phrases. It also includes the quoted advice, "Engage in conversations with your baby. Even though your baby can't speak, you communicate as if your baby is able to do that and you interpret her looks and her gestures". I think this relates to our discussion of how being accommodating to the baby is a critical part of "Baby Talk" / "Motherese".