Nancy doesn't like his name. He thinks it's a girl's name. None of the other kids think Nancy's name is strange, but Nancy is really self-conscious about it—so he's quiet and shy and never says anything. His only friend is a girl who goes to class on the twenty-third story of Wayside School.
Nancy and his friend are happy not knowing each other's names. They call each other "'Hey, You' or just plain 'You'" (28.4).
One day, Nancy's friend is late to class and he overhears her teacher call his friend "Mac." Mac is mortified that Nancy has found out, but Nancy calls after her and tells her his name: "I was ashamed to tell you my name" (28.13), she says.
They bond over their mutual hatred of their own names—Nancy was named for a favorite aunt, and Mac a beloved dog—and decide to trade names. Apparently the best way to do this is to spin around "one hundred times in opposite directions" (28.24) until they fall over.
When Nancy—now Mac—goes back to class, he doesn't feel shy anymore. He introduces himself as Mac, and everyone in the class starts to think that this trading names idea sounds pretty cool. They all decide to trade names with each other, spin around, and fall over, but… "when they stood up again, no one knew who anybody was" (28.43). Oops.
They argue over whose names are whose for over an hour, until someone finally figures out who Rondi is because she's missing two teeth. From there they start to figure out the other students, and also Mrs. Jewls, who's the oldest.
To avoid further confusion, everyone decides to keep his own name.
Mac and Nancy are the only ones who keep their new names, but since old habits die hard, whenever they get together they still call each other "Hey, You" or just plain "you."