This is an amazing short animation of a sack bag. A lot of traditional animators had used the idea of creating an animation for a simple sack bag for various reasons, one main reason is because they say, if you can give life to something as simple and plain as a sack, then you can breathe life into anything. That of course is what animation is all about, making something believable and give it life.
The key thing that seems to have come from watching this, reading various books, watching several bonus features on animated films and listening to many podcasts and talks, is that you need to understand what the character is about. The character needs a purpose or a reason for being, what does the character desire, need or feel. This will bring a basic understanding as to how the character may react and move, this is giving the viewer something to also base the character on and feel a greater connection too which makes it easier to understand and accept.
I may try animating the sack myself as it holds a lot of principles there to understand the levels within animation.
As it comments in the book ‘The Illusion of Life – Disney Animation‘ by Frank Thomas & Ollie Johnston,
“The famous half-filled flour sack, guide to maintaining volume in any animatable shape, and proof that attitudes can be achieved with the simplest of shapes.”