After working in the field of pop-up books for years, these two guys knew a lot about the art of paper engineering. They would have conversations as they were designing new books in which one would say to the other, "Why don't you do the "tent-thingie" on that page." Rather than go through life referring to tent-thingies, they came up with the idea of creating a book that defined the art of pop-up books in which they have created names for the different techniques they use. You learn all about folds and flaps and wheels and even "tent-thingies." For the young and old minds who love to understand how things work, here is the defining manual.