How do I make my GameObject "jello-y" so that it will mold and bounce?

In the game sushi cat, the cat's belly bounces around when it collides with something. How can I get a similar effect?

Sushi Cat: http://armorgames.com/play/5379/sushi-cat

Bones with spring or hinge joints attached.

http://forum.unity3d.com/threads/7629-Santaland

http://forum.unity3d.com/threads/7938-gooey-gumdrops-pyramid-performance