By the time she was 9 months old, my daughter Grace knew that she should raise her arms when I asked, "How big are you?" "So big!" I'd say, laughing as I pulled a clean shirt over her head. She couldn't have known I was referring to her size, but by a process of repetition and imitation she learned that the question meant it was time to put on a clean shirt.
At 16 months, she'd say, "Bo bee" -- her version of "So big" -- to indicate that she wanted to put on a shirt that she'd retrieved from the laundry basket. By listening, remembering, and imitating, she'd learned cause and effect, to connect words with actions.
By 18 months, Grace lifted her shirt to reveal her Buddha belly, an attribute that had been often praised by her parents. She spoke her first sentence: "I big belly." Although she still doesn't associate the question "How big are you?" with her size, it seems that in a separate process, she has learned the concept of "big," and accurately used it to describe her stomach.
Before a toddler learns to speak, we grasp for any clue to personality or intelligence, asking ourselves, "What do you suppose is going on in that little mind?"