One face, one decision
The baby cannot tell if the clear glass over the "deep" side is safe. Unsure, it looks up to the caregiver — and treats that emotion as information about the drop.
Only when it's ambiguous. On an obviously shallow surface babies cross no matter the face; over an obviously deep one they refuse regardless. The caregiver's expression steers the decision only in the uncertain middle — that is the signature of true social referencing.