My daughter has just discovered indefinite articles. For the past couple days, every "Mommy" and "Daddy" and "Kitty" has turned into "A mommy!", "A daddy!", "A kitty!" I think it's probably overanalyzing to think that she has just grasped either existential quantification or classification, but clearly she means something. She has been meaning things for a while, too, of course, but it's possible that this is her first communicated abstract idea. And even if this isn't, yet, it makes me realize that something soon will be.  

It's easy enough to see that a baby is a small person, taxonomically, because that's what they basically look like. But it's quite another leap to comprehend that an individual baby is actually inexorably becoming an individual person.