If you've used a View-Master Viewer (they're still around), you know how it works.  You look into the viewer with both eyes.  Each "slide" is really two images, two versions of the photo, illuminated from behind. The image that each eye sees is slightly offset from the other, which gives you the illusion of a single image in 3-D.  This utilizes an old technology called stereoscopic imaging or stereoscopy. While normally you need equipment to provide the focal point (such as the View-Master Viewer) for the images, it is possible to emulate the experience without anything other than your two eyes.  Using the image below, you can approximate the vintage View-Master experience.  Look at the slides, trying not to concentrate on either one in particular.  Relax your eyes. Then slowly cross your eyes.  It may take a while, but you will notice that the two images move toward each other, and then merge to create a third image in the middle.  This third image will appear to be in 3-D.  This is close to what you'd see viewing these photos in a View-Master Viewer.

Some people find this "forced" stereoscopy easy; others find it takes some practice.  (You'll find stereoscopic images available on the internet for a variety of subjects.)  Smaller images that are close together are easier to view in 3-D.  Also, don't practice this for too long or you'll get eye strain and perhaps a headache.  If your mother knew what you were doing, she'd probably warn you that your eyes might get stuck!  Safer to get a View-Master Viewer and some reels to enjoy this 3-D experience.