The persistence of vision is the phenomenon which is responsible for why we view animation.
When we see an object, the light reflected from it passes through the lens of the eye and is projected onto the retina in reverse. The information is then passed to the brain via the optic nerve where then the image is flipped the right way round.
When the object is removed from view, the image on the retina is removed after the amount of time it takes for the light to no longer reach the eye (which is almost instant). However, the image in the brain lingers for a little longer, approximately a 10th of a second.
An invention that demonstrates this idea is the 19th Century Thaumatrope.
This is a device made of a circular disc with two different drawings on either side. It is attached to two strings, and when it is spun fast (faster than a 10th of a second approx) then it will give the illusion of one image. This is because both images are being observed faster than the brain can distinguish the difference, and before the previous image can disappear.
Lots of other devices were invented from this idea including the Zoetrope and the Praxinoscope.