What happens to a caterpillar when it changes to a butterfly?

Somehow in conversation today the question came up if something like the Hulk would ever be possible. After a little thought I considered what happens in the metamorphosis of a caterpillar becoming a butterfly and the changes that occur. Then, the question came up about how that happens and I had an answer but I really didn't know enough to answer in more detail. Here is a cool site with a short article about what happens inside as a caterpillar changes into a butterfly. They used 3-D scanning to get the images internally and created an animation to show what happens!

Go to the article!