Baby talk.

I used to be amazed when I would talk to a kid that couldn't really talk yet, and their parents would know what they were saying.  It would go something like this:

Me: "Hey, buddy, what's going on?"

Kid: "Da!" (points).

Me: "Uhh, I don't really understand.  What do you want?"

Kid: "Da!" (points again).

Me: "Um, I still don't know what you're saying."

Kid: "Da!" (growing irritated).

Kid's Parent: "He wants you to go over there and pick up the red ball and throw it to the dog."

It was always a mystery to me how so much could be divined from so little.  

Now, though, Number One is saying quite a few words, all of them some variation of "Ba!" or "Da!" or "Ca!" and I'm starting to understand.

For example, "Ba!" when said when she's done eating dinner means, "I'm ready to go take my bath. Now!"

By contrast "Bo!" (with a short "o" like in "dog") almost always  means "I'm going to go get a book, and then you're going to read it to me. Now!"

"Ca!" usually just means "Cat!"