Trials: Accordion book, flip book, zoetrope, and thaumatrope. 

The above video is an example of a thaumatrope. It is two images, one on each side of a card spun with string between the thumbs that allows for the mind's eye to place both images on top of each other. 

The above video is a flip book. It is a series of images hole-punched and tied together. While most have the capability to view both images at the same time, mine is a Haley original so you have to work a little to see both images.