Books sure are changing these days, aren't they? In the past, you needed to head to the library or the bookstore to browse through physical books on the shelves to find a new adventure to check out or buy.

With today's modern technology, though, your next great read is just a few clicks away. Thanks to tablet computers and smartphones, combined with e-book technology, you can browse for new books in a web browser. When you find something you like, all you need to do is click, pay online, and download your book to start reading it instantly.

All that technology is great, but some people still like to hold a book in their hands and feel the pages turn beneath their fingers. This is especially true with movable books, such as pop-up books.

There's really no electronic substitute for enjoying a physical book that's also a work of art in and of itself. And that's certainly true of the subject of today's Wonder of the Day: the tunnel book.

Be sure to watch today's video if you've never seen a tunnel book. They're so cool! In the same category of movable books, such as pop-up books, tunnel books represent a unique meshing of storytelling with three-dimensional art.

Tunnel books are made up of a series of pages that are held together by folded strips of paper on each side. In fact, the sides of a tunnel book might make you think of an accordion. The overall effect of a tunnel book is to create the illusion of depth and perspective.

Tunnel books are “read" through a hole in the cover. Each page features openings that allow the reader to see through the entire book to the back cover. The images on each page work together to form a three-dimensional scene inside the book that helps to tell the story.

Tunnel books date all the way back to the mid-18th century, when they were originally called “peep shows." The concept of tunnel books was inspired by theatrical stage sets. If you've ever witnessed a live play with movable set pieces, it's easy to see how they inspired tunnel books.

The name “tunnel book" probably came from the fact that many of these types of books were created to celebrate the building of the tunnel under the Thames River in London in the mid-19th century. Throughout history, tunnel books have often been used to commemorate certain events. They've also often been sold as souvenirs at tourist attractions.

Tunnel books are alive and well today. Artist Carol Barton creates modern tunnel books that are very popular. Other artists have taken tunnel books to the next level by turning their accordion sides and front and back covers into canvases for additional artwork.

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