A flip book is a stack of slightly changing pictures. When the pages are turned rapidly an illusion of motion or morphing can be seen.
How to create a flip book?
You can use the corner of a book, post-it notes or a stack of index cards held together with binder clips.
A flip book is a lot easier to create if you see the previous drawing shining through. For books and post-it notes you could start your animation on the last page, put the previous page on one top of it and work your way up to the top.
If you work with loose index cards you can work bottom to top as you are animating, but later reorder the drawings to go top to bottom, because this is easier to flip with your thumb.
Using a light source underneath your flip book pages can make it a lot easier to see the previous drawing and even judge the Spacing is the distance an element travels between two frames of an animation. By increasing and decreasing the spacings... More between multiple pages. If you don’t have a light source, you could use a window.
Here is a very helpful flip book tutorial by Andymation: