What is the shortest complete sentence in English?

There are two main candidates for the shortest sentence in the English language. 

The first is the single word exclamation: Go! The challenger is I am.

My vote is for Go!. This has an active verb, an implied subject and a clear meaning. 

I am, as a sentence, only has meaning when used as an answer to a question: Who is going to the Moon this morning? I am.