When people hear the term pop-up book, they probably picture colorful storybook characters leaping from two to three dimensions at the flip of a page, or a clown whose tongue sticks out and arms wave at the pull of a tab in a children’s book.

But behind the colorful images and fun tales, pop-up books are complicated construction projects.

“Paper Engineering: Fold, Pull, Pop and Turn,” a new exhibit in the National Museum of American History, features the art of designing and creating “movables,” books that have moving parts.

The exhibit’s display of more than 50 interactive books, created between the 15th and 21st centuries, shows the various engineering methods of creating movable books. Visitors will learn that, in a traditional pop-up book, images are affixed to a page and, as the page opens, hinges connected to the paper pull at the cutout and raise the object, said Stephen Van Dyk, exhibit curator. Other movable objects in books include pull tabs, where figures move when pulled by a side bar; volvelles, or wheel charts that spin to reveal different images underneath; flaps that fold open; and accordion-style foldouts.

Many movables feature before-and-after themes or contrast differences, such as war and peace or earth and air. A book called “What Do You Get?” engineered by Tor Lokvig in 1968 asks riddle questions and reveals the answers when a tab is pulled.

The exhibit also uncovers the history and progression of interactive books. For one, children weren’t the only ones flipping pop-ups, spinning wheels and pulling tabs. In 1662, French philosopher, mathematician and physicist René Descartes, for example, used flaps in his anatomy book, “Renatus Des Cartes de homine,” to reveal the external and internal images of body organs. A copy of the book sits across the room from “Parascience Park: An Interactive Exploration of Your PSI Powers,” a book on parapsychology engineered in 2000.

Movable books predate Johannes Gutenberg’s printing press, Van Dyk said. The first known movable was a 1250 volvelle wheel used to track church holidays. And for most of the books’ pop-up, flip-out and fold-over history, the subjects of relevance were science, astrology and mathematics.

“It wasn’t until the 19th century that we really saw books used to entice children into reading and learning,” Van Dyk said.

Throughout movable books’ history, one thing has remained the same: They’re still a hand-assembled craft. Each pop-up and flap is cut from a sheet of printouts, assembled and glued by hand. The process usually takes 18 months from “concept to product,” Van Dyk said, with collaborative work between book authors, designers and paper engineers.

But the long, tedious process has not hindered recent paper engineering developments. Van Dyk believes a “great age of paper engineering is beginning.” Birthday and holiday cards, for example, now include lights and music, he said.

“They’ve become a portable form of entertainment and art,” Van Dyk said, adding that he expects new designs to continue to unfold.

“Paper Engineering” will remain open through Sept. 1, 2011.