One of my favorite things that was shown to me during my elementary age years was an optical illusion. It was the very famous Rubin Vase (pictured below) which you can see in either two ways - as a vase or as two faces that are facing each other.
photo manipulation before the digital age. 'american school man on rooftop with eleven men in formation on his shoulders' by unknown artist, ca. 1930 gelatin silver print collection of george eastman house photo courtesy: the metropolitan museum of art