Why does Marshall believe the Oracle can locate Security?

Security is Marshall's stuffed octopus. In chapter 18, Marshall is very upset because he can't find Security. Marshall believes that he left it in Egypt. All of the searching comes up empty, and Marshall believes that he should ask the oracle where the stuffed animal is. He believes the oracle will know the answer because of what he learned and experienced in the previous two chapters. In chapter 16, the kids learn that oracles were people that gave wise advice and had special knowledge because it came from the gods. Often, the oracles were able to predict the future.

I mean all those other oracles weren’t just kid stuff. Even kings and generals and all sorts of other adults used to go for this oracle stuff, didn’t they?

The kids decide that their Egypt needs an oracle, so they create one for their game, and they come up with a method by which to ask it questions and receive answers. Chapter 17 is about the answer that the kids get to one of the questions that they ask. Marshall is convinced that the oracle business is real and that the oracle will be able to tell him where Security is. Unfortunately, the oracle's answers are being faked by Toby.

When April and Melanie wandered over—and registered exaggerated surprise to find him there—he got right to the point. “Look,” he said, “I was the one who wrote those answers. I was the oracle. But I don’t know where Marshall’s old octopus is. What’re we going to do?”