the thaumatrope [ thaw-muh-trohp ]

A thaumatrope was a popular Victorian era toy. It was a disk or card with a picture on each side is attached to two pieces of string. When the strings are twirled quickly between the fingers the two pictures appear to combine into a single image due to persistence of vision...remember the “bird in the cage”? 

There is a shape, or profile, hidden within the two drawings and is only entirely visible once the disk is spinning. During the spinning action, the detail of the two drawings will blur and allow the white of the negative space in each drawing to become the prominent visual feature. So if you spin the disk fast enough and allow your eyes to relax, eventually you will see the shape of a long-neck, even-toed ungulate of the African savanna.

...spinning disc reveals giraffe.