What is the shortest complete sentence in English?

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

1. Go!
2. I am.

Go! arguably has the the strongest case. It has an active verb (t0 go), an implied subject (second person singular or plural) and a clear meaning. 

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