How do crawlers and walkers learn?

Marcus picks up a book with cardboard pages and a duck on the cover. He puts the book in his mouth for a moment, then waves it in the air.

Marcus looks at his big sister, Maria, and makes noises that sound a lot like words. Maria says, "Do you want to read? Bring the book to me. We can read together."

With book in hand, Marcus crawls to Maria. She lifts Marcus into her lap and holds the book so that he can see it. She points to the duck on the cover. "That's a duck. Let's see what's inside."
Marcus turns the page. He pats the picture and says something that sounds like words. "That's right," says Maria. "The baby is in the bathtub."

After looking at a few more pages, Marcus squirms and wiggles. "Okay," says Maria. "Have you read enough? Let me help you down." She puts Marcus on the floor and he crawls away.