Why does jelly wobble?

This would be used to demonstrate still jelly, sort of like before the storm. 
The studio gives the jelly, great clarity an application to how good jelly is. 

This was the verdict after editing in photoshop. I  burnt in the background so you could fully see the different exposures of the Jelly wobbling. This gives children a clear understanding and example of a wobbly Jelly, but photographed in a playful manner.