Can anyone help me out / point me in the direction of a tutorial of how to make a object look like its filling up.

Example - http://www.holycow.uk.com

Their cow graphic fills up as a preloader to the images and it looks cool!
How did they do it? Ive been looking for ages but not getting anywhere or just not looking in the right place!

Any help would be great

Cheers
creative.watson