Back in the 1990s there was a popular series of picture books with an unusual twist. When you looked at the pictures on the surface you would see an intricate two-dimensional image that was mildly interesting. But if you learned how to look past the surface, suddenly a much richer and deeper three-dimensional image would reveal itself. Seeing it took extra work - some people would get very frustrated when they couldn't get it to appear - but the reward made it worth the effort.