One common type of computer animation is made of a number of seperate pictures called CELS. Each picture is slightly different from the others. When all the pictures are shown rapidly, one after the other, it appears as though there is a moving picture.

The more cels in the animation the more realistic it becomes. The animation shown here is made of four cels ... it's not very good.

In this animation the individual cels were drawn on paper and then captured onto the computer with an ordinary flatbed scanner. A piece of software was then used to compile the cels into one animated picture.