"If the Wonders of the World hadn't stopped at seven, Cobble's Knot would have been number eight" (19.1).
Can a knot really be this special? Well, this one is is. It isn't just any knot;it's a knot with "more contortions, ins and outs, twists and turns and dips and doodles than the brain of Albert Einstein himself" (19.7).
So how does this knot, this knot that has "defeated all comers for years, including J.J. Thorndike, who grew up to be a magician, and Fingers Halloway, who grew up to be a pickpocket" (19.7) come to meet one Maniac Magee?
Maniac isn't spending a day trying to untangle Cobble's Knot just for kicks. He's not even doing it for the free pizza (one a week for a whole year!) He's doing it cause Amanda asked (told?) him to. Let's hear from Amanda herself: "So you see, if you go up there and untie Cobble's Knot—which I know you can—you'll get your picture in the paper and you'll be the biggest hero ever around here and nooo-body'll mess with you then" (19.9).
So, let's think about why Amanda thinks that solving this knot will solve all Maniac's problems. Maniac is special, obvs. He can do things others can't, like live off the grid, hit an inside-the-park homerun frog, and catch a touchdown pass one-handed. And these actions add to his legend. Amanda figures that, if he solves the knot, his legend will grow so big that he'll somehow just rise above all the race-related tensions.
And, gig surprise, Maniac manages to untangle the knot: "a huge roar went up, a volcano of cheers. Cobble's Knot was dead. Undone. Gone. It was nothing but string" (20.21). It's a huge celebration on Cobble's Corner, and yup, Maniac is the hero. Whew—problem solved.
Not so fast. The magical happy ending doesn't quite appear. In the middle of the celebration, "one person wasn't yelling: Amanda Beale. She was holding one of the homemade confetti scraps, gaping at it" (21.4).
Mixed up in the celebration is the vandalism of Amanda's most prized possession: The encyclopedia A. This is a message to Amanda, and to Maniac: you're still not welcome here.
So a knot can't solve everything. Untangling a knot can't make people like you, though it may make lots of people cheer. Because, ultimately, a knot is a thing, and things can't change people. People change people.
Maniac takes off after the Encyclopedia A is destroyed because things haven't changed—Cobble's Knot hasn't fixed everything. Maniac (and Amanda) thinking that untangling the knot will make everything better shows their youth and inexperience. Living at 728 Sycamore will probably never be easy, and Maniac will probably need to deal with prejudice and hatred as long as he lives with the Beales.
One way of thinking about it is this: Cobble's Knot symbolizes all the tricky, knotty problems that come along with living in this tricky, knotty world. And being able to unravel a string knot doesn't mean you're ready to unravel the world. Figuring out Cobble's Knot is just one step on Maniac's journey to get tough enough to be able to stay with the Beales. He's just knot there yet.