Opal's explanation to her grandson isn't quite right. "Funner" is substandard, and she's right to correct him, but saying that "funner" isn't a word is wrong. Any combination of sounds with a consistent meaning is a word, even if we don't like it. (Ever hear a child talk about his "binky"? It's not in the dictionary now, but someday it might be.) Our ever-changing language is more flexible than many of us like to think.