This sentence appears in Chapter 37, which begins by recounting "a series of heroic feats" that Maniac performs. But why does Maniac perform all these feats? We find out in this paragraph:

To Russell and Piper McNab, it was a case of boosting their importance ever higher in the eyes of the other kids. Was it not at the brothers' direction that Maniac Magee performed these deeds? And who after all is the more amazing, the lion or the tamer?

From these sentences, it becomes clear that the McNabs are the ones daring Maniac to perform his feats of heroism. We can figure out what the author means by that last sentence by thinking about what we know about lions and lion tamers. Basically, a lion tamer is the one that gets the lion to perform tricks. Since Maniac is the one doing the tricks, he must be the lion. Since the McNabs are the ones daring him, they must be the tamers.

While most of the town thinks of Maniac as amazing for being able to outrun a freight train and walk through the rat-infested dump, the McNabs think of themselves as the amazing ones. Like lion tamers, they're the ones who got the lion to do the tricks in the first place.

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