Illusion - 'Persistence of Vision'

Animation in all its forms is actually an illusion. It is a trick of the eye, which is known as the 'Persistence of Vision'

The human eye sees and retains an image or frame for just a fraction of a second. So when that image is replaced with another image, slightly different and straight afterwards, movement appears to occur to the human eye.

We believe that we are seeing a movement or film, however, what is really seen is a long succession of static images, at a rate of 24 frames per second, and therefore the eye is deceived.