The feeling of weight is something extremely important to your animations, making them look believable. When looking at shapes on a screen (be they realistic, cartoon, or abstract), the way they move frame to frame is what makes the audience perceive their mass.
The weight lifting exercise is one of the most repeated in animation schools worldwide. This is where animation students can test and learn the body mechanics necessary to express the feeling of weight with animated shapes. In this recipe, our character is dressed as a thief and will try to take a safe full of money with him.
It's a good idea to act it out yourself (maybe recording it, if you have a webcam or better equipment) to figure out the body mechanics involved. As with ...