"People don't really understand the nature of reading. They feel that reading is a skill, that it's transferable, so once you're a good reader, you can read anything that's put in front of you," says Daniel Willingham, a University of Virginia cognitive psychologist who focuses on K-12 education. "But that's only true for decoding—what you learn until grade three or four. After that, when you see good readers versus poor readers, what you're looking at is mostly differences in the knowledge that kids bring to the reading. It's easy to read something when you already know something about the topic. And if you don't know about the topic, it's utterly opaque to you."

That's why children should read newspapers and magazines, texts about nature and technology, and biographies—genres that increase real-world knowledge. This is especially important for poor children, who may not be exposed to as much "background" information at home: the random vocabulary, facts, and associations that make it easier to do well on tests like the NAEP and SAT, and to succeed in the workplace.