Stereograms is another example of how the brain can be confused. Apart from being great fun to be able to 'see' the hidden picture, it also is a very good brain exercise. Not every person can see them as easily as others, but nearly everybody can and only people with visual impairments can't see them.. The harder you have to look, the more you engage your brain, and that is maybe a good thing.
To see the pictures, either look at them cross-eyed or by looking straight on. In both cases try and look 'through' the picture as if you are looking at something behind the picture.
Science and technology revolutionize our lives, but memory, tradition and myth frame our response.