Crazy Question

For you children's librarians out there (or those with a remarkable knowledge) I've a crazy question. You know those books where you flip pages and change the image before you? For example, the image is of a face and you can flip to change the eyes, nose and mouth, thereby creating a wide variety of different combinations? Right. What are those books actually called? Is there a term for them? Because, to be frank, I haven't a clue.