Traditional Principles of Animation/Squash and stretch
The principle of Squash and Stretch implies that no matter how much an object deforms, it must always maintain the same apparent volume. Proper use of Squash and Stretch illustrates just how dense or rigid an object is. All real world objects, unless extremely hard, exhibit some form of squash and stretch.
The classic example is the bouncing ball. The ball squashes as it hits the ground, then stretches on its way up and then back down. When an object squashes along one axis (direction), it automatically expands along the other two. It is up to you, as the animator, to ensure that any deformation along on axis is offset be an opposite deformation along other axes maintaining a constant volume.
Consider the way a pillow changes shape as it's tossed across the room. The shape deforms, but does the volume of the pillow change? No matter how contorted the pillow becomes, it still maintains the same weight and amount of stuffing.