When it has a subject, verb, punctuation, capitalization, a predicate, and a complete thought. A sentence is a sentence when it is handed down by a judge (or a magistrate).
A sentence is a complete clause with a subject and a finite verb. A sentence fragment is not a complete clause, but is part of a clause (often lacking the verb to make a full sentence). For instance,...
It's not possible, but the closest thing you can get is "It's how you are." I'll try my best to think of the answer, even though so far it seems impossible.