Heiji grinned widely. "And there's one other thing -- it should be a code that we can figure out without ever talkin' about it."

That's right, Heiji wanted to develop this code without ever discussing it in words, because of course someone might be listening. Pointlessly hard challenge? MAYBE. But just the effort that went into thinking one up was almost as rewarding as the end project itself. And could it not be said that codes were basically puzzles?