Differences are invisible until discovered. A common picture riddle found in children’s coloring books includes a number of seemingly identical snow men (or teddy bears, men in raincoats with umbrellas, etc.), one of which has a slight difference. Which of these is not like the others? The child looks at the three or four snowmen bewildered, even frustrated, until aha! This snowman is missing a button. The child circles the unfashionable snowman, and turns the page.