Cels, short for "celluloid," are transparent sheets used in the animation process. They serve as a medium for artists to paint characters and scenes, which are then layered over a static background. This technique allows for the creation of fluid motion in animated films and television shows. In traditional animation, each cel represents a single frame of movement. When these cels are photographed in sequence, they create the illusion of motion. This method was widely used in classic animations, such as those produced by Walt Disney, before the transition to digital animation techniques.