Animation is a simulation of movement created by displaying a series of pictures or frames rapidly to give the illusion of continuous motion. It works by taking advantage of the persistence of vision, where the human eye retains an image briefly after viewing. A series of images changed slightly and rapidly will seem like continuous motion. Early pioneers included Walt Disney, who created the first animated film with sound in 1928 and the first full-length animated color film in 1937. Traditional cel animation involved drawing and painting characters on transparent sheets called cels that were photographed and layered over backgrounds.