The squash and stretch is considered by some to be the most important principle of animation, and is based on the observation that only objects that are stiff (like doors, windows or candlesticks) remain inert or maintain their shape during motion, while objects that are not (like animals or humans) will change in shape while maintaining their original volume, or mass. To give an example, think of the fundamental difference between solids and liquids; the former must maintain a certain shape, while the latter is more flexible and can fit itself into any container. This principle also makes up part of the basic training which all Disney animators needed to go through, by animating a ball that would compress itself (squashing) as it hit the ground, before elongating as it bounced off again (stretching), and this was done back in the 1930s where both processes were exaggerated by the animators.