Stereograms
A stereogram is a picture within a picture. Inside each image is a hidden object which appears in 3D when viewed correctly. Bringing the stereogram image really close to your eyes (until you touch it with your nose), your eyes cannot focus on the image and they look somewhere behind the image. While trying to keep the eyes off focus and by slowly pushing the image away from you, you will see the hidden image. Interesting, don’t you think?