An Easter Egg

An easter egg is the name for something hidden inside a computer game, program or movie which can be discovered if you know the secret - such as a combination of keystrokes or other actions. Yet the concept of something hiding inside media is not new. In what's called a hidden fore-edge painting, whose earliest example is a Bible from 1651, entire paintings were hidden within the edges of the pages of books. An artist would fan out the book's pages and clamp them down, and then paint an image on the fanned-out edges. Then the artist would close the book and cover the edges in gilt to hide the painting. (If the edges were not covered in gilt, you'd see a squashed version of the original painting on the edges.)
When a reader read the book, he or she would very likely not notice that every page contained a thin slice of a painting. But when the pages were fanned out...