One of the most important lessons we learn during Greg and Maura's argument in Andrew Clements' Lunch Money is we all have the freedom to express ourselves. Greg starts the argument because he feels Maura has stolen his idea; however, as Mr. Z points out, Greg doesn't truly have possession over the idea of making minibooks, just as no single person owns the idea of writing books in general.
Soon after Greg develops and begins to sell his first mini-Chunky Comic book, Greg discovers that his longtime rival, Maura, has developed her own brand of minibooks titled the "Eentsy Beentsy Book" series, and the first mini-comic book she created for the series is titled The Lost Unicorn. Although he sees that Maura's book is really more of a mini-picture book than a mini-comic book, he feels she is copying and profiting from his own idea, so he marches up to her and accuses her of being a thief.
Later, in a conference, their math teacher, Mr. Z, points out that a small book illustrated with pictures is really only "an idea," not Greg's own idea; in fact, it's a very classic and historic idea, just as the Sumerians developed the very classic and historic idea of "carrying tens or hundreds over into the next place column" when doing addition (71-72). Just like the Sumerians can't accuse other people of plagiarism for using that addition custom, no one can accuse anyone else of plagiarism for writing a book with illustrations or for writing any book at all. As Mr. Z further explains to Greg, Greg can only really accuse Maura of stealing his idea if she had invented a character for her story that was just like the character Creon in Greg's story. Mr. Z's arguments show how important it is for people to feel they have freedom to express themselves, and Greg is wrong to accuse Maura of stealing just because she expressed herself in a book of a similar size and length to his book.