Magic eye books and posters became trendy for a sec when they first came out in the mid-90s, and everyone was naturally fascinated with those strange static pictures only certain people could see.
The images look like noise fields yet "magically" contain a 3D image within, and people who have figured out how to see the hidden image love teaching others how to "look without looking" or "use deep focus".
But some people can't see the hidden image no matter how hard they try, thinking Magic Eye pics are just a bunch of colored dots on the page when the real problem is with the viewer's stereo vision:
To view 3D stereo images, your peepers have to work together as a coordinated team. If they're not pulling together, you're going to have some glitches in your binocular (two-eyed) vision or stereo vision (where the two slightly different views from your eyes are combined in the brain). A number of things can cause binocular and stereo vision impairment — most commonly, deviations or misalignments of one or both eyes ("crossed eyes" or "wall eyes"), situations where one eye is dominant because visual stimulation either transmits poorly or not at all from the other, astigmatism or cataracts. If you think you have an eye problem, go see an eye doctor who can test and treat your stereo vision.
And for those who are curious the top image is just waves, the second image is dolphins and beach balls, and the last and hardest one to see is puppies and a Christmas tree.