A spring, a spring, a marvelous thing!
It’s the shadow illusion that it creates that’s interesting here. The shadows created by the slinky look as though they are bent, almost in the shape of hearts. Of course, it the shape of the open pages of the book that are creating this shadow illusion. The shadows of the oversize spring are falling along the pages, which are curved, making the shadow appear curved as well.
If you like shadow illusions too, head over and check out my favorite shadow illusion—a shadow falling over a cliff!