(from More Joy of Mathematics , by Theoni Pappas, pg. 13).

On a bookshelf is a six-volume set of books, arranged in order from
volume 1 to volume 6. Suppose each of these books is 2.5" thick,
including the .25" thick covers. A bookworm is 1" long. The bookworm
eats in a straight horizontal line beginning at the front cover of the
first volume and ending at the back cover of volume 6. How many inches
did the bookworm travel?

The bookworm travels 10.5". The path the bookworm took was to eat the front cover of volume 1, all of volumes 2, 3, 4, and 5, and the back cover of volume 6. The books are arranged as follows:

The front cover of volume 1 and the back cover of volume 6 are on the "inside" of the stack.