What is the secret ingredient of the secret ingredient soup?

In the movie, Kung Fu Panda, the secret ingredient in secret ingredient soup is never revealed. https://skepticink.com/smilodonsretreat/2015/12/28/there-is-no-secret-ingredient/

The secret ingredient is nothing:

Mr. Ping: The secret ingredient is... nothing!
Po: Huh?
Mr. Ping: You heard me. Nothing! There is no secret ingredient.
Po: Wait, wait... it's just plain old noodle soup? You don't add some kind of special sauce or something?
Mr. Ping: Don't have to. To make something special you just have to believe it's special.
[Po looks at the scroll again, and sees his reflection in it]
Po: There is no secret ingredient...

http://www.imdb.com/title/tt0441773/quotes