Why do cartoon characters squash, stretch and wind up before they move?

Because these tricks make a flat drawing seem to have weight and to obey physics. Disney's twelve basic principles of animation, set out by the animators Ollie Johnston and Frank Thomas in their 1981 book The Illusion of Life, were meant to make cartoon characters seem to obey the basic laws of physics, and they also dealt with emotional timing and character appeal. They grew out of the work of Disney animators from the 1930s onwards.

Squash and stretch is the clearest example. It gives a sense of weight and flexibility to drawn objects: a ball flattens as it hits the ground and lengthens as it bounces up. The rule that keeps it believable is that the object's volume does not change, so a ball stretched tall must also get thinner. To teach it, a half-filled flour sack was dropped on the floor or stretched by its corners, and it kept its overall volume. The animators also studied photographs of athletes from newspaper sports pages to see how a body bends in motion.

Anticipation is the wind-up. It prepares the audience for an action and makes it more realistic: a dancer bends the knees before jumping, and a golfer swings the club back before the swing. After the action comes follow through: loosely tied parts of a body keep moving after the character has stopped, then get pulled back.

Timing completes the trick. An object's weight determines how it reacts to a push, and a lightweight object reacts faster than a heavy one.