Knox starts the book enjoying a nice leisurely pace to the word games: "Fox in socks / on box on Knox" (5.1-2). You won't find a cheaper item in the tongue-twister catalogue. And you can tell by the smile on his face that Knox is having a good time.
Then things get difficult. Once Fox plays his tricks with chicks and clocks, Knox complains that his "tongue isn't / quick or slick, sir" (13.3-4). Fox promises easier things to say, but with each successive tongue-tangler, Fox ups the difficulty from medium to hard to insane. And with each upgrade, Knox becomes more confused, more upset, and more disparaged about how his "poor mouth can't / say that" (36.3-4).
Just like Charlie Brown, Gordon Freeman, and Dr. Watson, Knox is the everyman character. To put it another way, he is the surrogate character for the reader. As the rhymes get trickier for Knox, they get trickier for us as well. When Knox complains that "[t]hat's not easy" (19.1), so do we. And when Knox starts to get flabbergasted and lose confidence in himself, chances are you're right there with him, especially if this is your first reading.
But Knox's arguments are a bit misleading. For example, when Knox says:
Pig Band! Boom band!
Big band! Broom band!
My poor mouth can't
say that. No, sir. (36.1-4)
he is, in fact, saying it. Knox's statements are the very tongue twisters he claims he can't utter.
The same goes for the reader. Going back to our surrogate talk, the reader may feel like Knox, believing the tongue twisters too difficult. But as they read the words, they prove both themselves and Knox more capable than initially imagined.
This feeling of connection between Knox and the reader continues all the way up until the end of the book. Unlike poor old Charlie Brown, Knox does get to kick his metaphorical football when he puts a lyrical smack down on Fox:
[…] a tweetle beetle
noodle poodle bottled
paddled muddled duddled
fox in socks, sir! (57.1-5)
He then walks away from a dazed Fox, happy and thanking him for "a lot of fun" (58.4). In the same vein, the reader picks up on Knox's mood at the end of the book. Does it matter that the reader messed up on a few lines here or there? Not at all. We have met Fox's verbal challenge, we've seen it though, and we had a lot of fun doing so.